DescriptorExtractor: added defaultNorm() to return default norm type.

Added the defaultNorm() method to the DescriptorExtractor class. This method returns the default norm type for each descriptor type. The tests and C/C++ samples were updated to get the norm type directly from the DescriptorExtractor inherited classes.

This was reported in feature report #2182 (http://code.opencv.org/issues/2182). It will make it possible to get the norm type usually applied matching method for each descriptor, instead of passing it manually.

samples/cpp/tutorial_code/features2D/SURF_descriptor.cpp

@@ -49,7 +49,7 @@ int main( int argc, char** argv )
   extractor.compute( img_2, keypoints_2, descriptors_2 );
 
   //-- Step 3: Matching descriptor vectors with a brute force matcher
-  BFMatcher matcher(NORM_L2);
+  BFMatcher matcher(extractor.defaultNorm());
   std::vector< DMatch > matches;
   matcher.match( descriptors_1, descriptors_2, matches );