Modify Base64 functions and add test and documentation

Major changes:

- modify the Base64 functions to compatible with `cvWriteRawData` and so
on.
- add a Base64 flag for FileStorage and outputs raw data in Base64
automatically.
- complete all  testing and documentation.

modules/core/include/opencv2/core/core_c.h

@@ -1976,8 +1976,19 @@ CVAPI(void) cvSetIPLAllocators( Cv_iplCreateImageHeader create_header,
 
 The function opens file storage for reading or writing data. In the latter case, a new file is
 created or an existing file is rewritten. The type of the read or written file is determined by the
-filename extension: .xml for XML and .yml or .yaml for YAML. The function returns a pointer to the
-CvFileStorage structure. If the file cannot be opened then the function returns NULL.
+filename extension: .xml for XML and .yml or .yaml for YAML.
+
+At the same time, it also supports adding parameters like "example.xml?base64".
+@code
+    CvFileStorage* fs = cvOpenFileStorage( "example.yml?base64", 0, CV_STORAGE_WRITE );
+@endcode
+it's exactly the same as
+@code
+    CvFileStorage* fs = cvOpenFileStorage( "example.yml", 0, CV_STORAGE_WRITE_BASE64 );
+@endcode
+
+The function returns a pointer to the CvFileStorage structure.
+If the file cannot be opened then the function returns NULL.
 @param filename Name of the file associated with the storage
 @param memstorage Memory storage used for temporary data and for
 :   storing dynamic structures, such as CvSeq or CvGraph . If it is NULL, a temporary memory
@@ -1985,6 +1996,7 @@ CvFileStorage structure. If the file cannot be opened then the function returns
 @param flags Can be one of the following:
 > -   **CV_STORAGE_READ** the storage is open for reading
 > -   **CV_STORAGE_WRITE** the storage is open for writing
+      (use **CV_STORAGE_WRITE | CV_STORAGE_WRITE_BASE64** to write rawdata in Base64)
 @param encoding
  */
 CVAPI(CvFileStorage*)  cvOpenFileStorage( const char* filename, CvMemStorage* memstorage,
@@ -2162,7 +2174,7 @@ the file with multiple streams looks like this:
 @endcode
 The YAML file will look like this:
 @code{.yaml}
-    %YAML:1.0
+    %YAML 1.0
     # stream #1 data
     ...
     ---
@@ -2187,6 +2199,46 @@ to a sequence rather than a map.
 CVAPI(void) cvWriteRawData( CvFileStorage* fs, const void* src,
                                 int len, const char* dt );
 
+/** @brief Writes multiple numbers in Base64.
+
+If either CV_STORAGE_WRITE_BASE64 or cv::FileStorage::WRITE_BASE64 is used,
+this function will be the same as cvWriteRawData. If neither, the main
+difference is that it outputs a sequence in Base64 encoding rather than
+in plain text.
+
+This function can only be used to write a sequence with a type "binary".
+
+Consider the following two examples where their output is the same:
+@code
+    std::vector<int> rawdata(10, 0x00010203);
+    // without the flag CV_STORAGE_WRITE_BASE64.
+    CvFileStorage* fs = cvOpenFileStorage( "example.yml", 0, CV_STORAGE_WRITE );
+    // both CV_NODE_SEQ and "binary" are necessary.
+    cvStartWriteStruct(fs, "rawdata", CV_NODE_SEQ | CV_NODE_FLOW, "binary");
+    cvWriteRawDataBase64(fs, rawdata.data(), rawdata.size(), "i");
+    cvEndWriteStruct(fs);
+    cvReleaseFileStorage( &fs );
+@endcode
+and
+@code
+    std::vector<int> rawdata(10, 0x00010203);
+    // with the flag CV_STORAGE_WRITE_BASE64.
+    CvFileStorage* fs = cvOpenFileStorage( "example.yml", 0, CV_STORAGE_WRITE_BASE64);
+    // parameter, typename "binary" could be omitted.
+    cvStartWriteStruct(fs, "rawdata", CV_NODE_SEQ | CV_NODE_FLOW);
+    cvWriteRawData(fs, rawdata.data(), rawdata.size(), "i");
+    cvEndWriteStruct(fs);
+    cvReleaseFileStorage( &fs );
+@endcode
+
+@param fs File storage
+@param src Pointer to the written array
+@param len Number of the array elements to write
+@param dt Specification of each array element, see @ref format_spec "format specification"
+*/
+CVAPI(void) cvWriteRawDataBase64( CvFileStorage* fs, const void* _data,
+                                 int len, const char* dt );
+
 /** @brief Returns a unique pointer for a given name.
 
 The function returns a unique pointer for each particular file node name. This pointer can be then

modules/core/include/opencv2/core/persistence.hpp

@@ -311,7 +311,10 @@ public:
         FORMAT_MASK = (7<<3), //!< mask for format flags
         FORMAT_AUTO = 0,      //!< flag, auto format
         FORMAT_XML  = (1<<3), //!< flag, XML format
-        FORMAT_YAML = (2<<3)  //!< flag, YAML format
+        FORMAT_YAML = (2<<3), //!< flag, YAML format
+
+        BASE64      = 64,     //!< flag, write rawdata in Base64 by default. (consider using WRITE_BASE64)
+        WRITE_BASE64 = BASE64 | WRITE, //!< flag, enable both WRITE and BASE64
     };
     enum
     {
@@ -354,7 +357,9 @@ public:
        Extension of the file (.xml or .yml/.yaml) determines its format (XML or YAML respectively).
         Also you can append .gz to work with compressed files, for example myHugeMatrix.xml.gz. If both
         FileStorage::WRITE and FileStorage::MEMORY flags are specified, source is used just to specify
-        the output file format (e.g. mydata.xml, .yml etc.).
+        the output file format (e.g. mydata.xml, .yml etc.). A file name can also contain parameters.
+        You can use this format, "*?base64" (e.g. "file.xml?base64"), as an alternative to
+        FileStorage::BASE64 flag. Note: it is case sensitive.
     @param flags Mode of operation. One of FileStorage::Mode
     @param encoding Encoding of the file. Note that UTF-16 XML encoding is not supported currently and
     you should use 8-bit encoding instead of it.

modules/core/include/opencv2/core/types_c.h

@@ -1669,6 +1669,8 @@ typedef struct CvFileStorage CvFileStorage;
 #define CV_STORAGE_FORMAT_AUTO   0
 #define CV_STORAGE_FORMAT_XML    8
 #define CV_STORAGE_FORMAT_YAML  16
+#define CV_STORAGE_BASE64       64
+#define CV_STORAGE_WRITE_BASE64  (CV_STORAGE_BASE64 | CV_STORAGE_WRITE)
 
 /** @brief List of attributes. :