work on 3D ROI.  Everything seems to be working (poly2mask, mask2poly, etc.) and inverting appropriately.

Might want to re-visit some of the (x,y,z) vs. (z,y,x) conventions as well as cases in which user passes in 2D polygons or mask to ensure behavior is intuitive
Merge branch '3D_SIMA' of github.com:losonczylab/sima into 3D_SIMA