Create di.span.patch.r

Dichromatism Measurement 1: Patch-by-patch volume centroid distance (span)

Measures distance between the centers of the convex hulls created by the set of spectra for each male and female patch.

Adds together the patch-by-patch distances for total color distance and divides by number of patches. 

If you are using additional patches beyond a standard set, make sure your results are correlated strongly with those procured with only your standard set (e.g. if you have 9 patches for some birds and a standard set of 5, you want to make sure that the additional 4 patches are not skewing the final measurement after averaging).