Fix for spurious assert in cv::compareHist

When comparing histograms that look like multi-channel images (e.g a 3D histogram, of 4x4x4 bins, might appear as a CV_32FC4 matrix), cv::compareHist would complain because it was expecting the matrix type() == CV_32F. Now we test matrix depth() == CV_32F instead.
Take account of multi-channel matrices when comparing histograms

Ensure that all channels are considered when comparing multi-channel histograms
Merge pull request #856 from ericgreveson:master