Add priority to voting resample module

Following what is done for PoseLabelmap, we add high and low precedence
labels to help with the resampling. The high precedence labels are always
picked over normal labels. Low precedence labels are only picked if the
aren't any other labels around.
Integrate VotingResample high and low precedence to the workflow

In this case, the high precedence labels are in order (remember that order
matters) skin, bone and muscle. The lowest (and only) low precedence label
is of the background.
Fix VotingResample border issue

Because the radius was constant, the output image wouldn't necesseraly have
the right label when resampling with a high spacing. It also assumed that
the image was uniformely spaced in 3D which might not be the case.
This fix bases the radius on the output image spacing thus taking into
account all the necessary voxel for voting.
Automatically adapt the VotingResample scale in the workflow

This follows what is done in compute armature weight.