voxel_vote is slow

[bcdarwin]

Currently voxel_vote makes len(dimension 0) * n_files calls to getHyperslab, resulting in a quite a number of I/O syscalls. It would be better to read in each file at a time and maintain a (sparse) label histogram at each voxel.

(One minor question is how to store the histograms, e.g. some sparse array format with a channels dimension for the label histograms seems appropriate, but it might be necessary to determine the maximum label in advance.)

[gdevenyi]

we did a sort of histogram thing which also enabled counting making weighted voting,
https://github.com/CoBrALab/antsRegistration-MAGeT/blob/master/bin/simpleitk_weighted_vote.py