roi.py: Added slicing_to_string()
OpCompressedUserLabelArray: Added Projection2D slot (needs optimization)
OpCompressedUserLabelArray: Vastly improved performance by ignoring missing blocks.
OpCompressedArray: Projection2D colors are inverted so that objects in low-numbered slices occlude objects in high-numbered slices.
memory management: Use .total - .wired on Mac/Linux.
See #699
Merge branch 'master' into multiprocess-hdf5