Bug fix: corrected a bug in HOGDescriptor::detectMultiScale()

The detectMultiScale did not group weights along with ROIs. A new protected method is added to HOGDescriptor structure to handle weight grouping, instead of using the generic groupRectangles() function in cascadedetect.cpp. A class definition had to be moved out of cascadedetect,cpp into cascadedetect.hpp to accommodate the change.

Fixes issue #3021 (http://code.opencv.org/issues/3021)