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/gpu/surf_keypoint_matcher.cpp

@@ -62,7 +62,7 @@ int main(int argc, char* argv[])
     cout << "FOUND " << keypoints2GPU.cols << " keypoints on second image" << endl;
 
     // matching descriptors
-    BFMatcher_CUDA matcher(NORM_L2);
+    BFMatcher_CUDA matcher(surf.defaultNorm());
     GpuMat trainIdx, distance;
     matcher.matchSingle(descriptors1GPU, descriptors2GPU, trainIdx, distance);