Fix missing format when writing Algorithm-based objects

Added a writeFormat() method to Algorithm which must be called by the
write() method of derived classes.

modules/video/src/bgfg_KNN.cpp

@@ -183,6 +183,7 @@ public:
 
     virtual void write(FileStorage& fs) const
     {
+        writeFormat(fs);
         fs << "name" << name_
         << "history" << history
         << "nsamples" << nN

modules/video/src/bgfg_gaussmix2.cpp

@@ -286,6 +286,7 @@ public:
 
     virtual void write(FileStorage& fs) const
     {
+        writeFormat(fs);
         fs << "name" << name_
         << "history" << history
         << "nmixtures" << nmixtures