added include/exclude properties on mask, implement data_valid
add WCS dropdegenerate tool
degeneracy dropping, minimal data indexing
two more manipulation routines
swapaxes used in spectral_cube now
drop degenerate in WCS too
shape returns _data.shape
WCS tests
added docstrings, dropped dropdeg from io
Merge pull request #3 from keflavich/minimal_mask_implementation

added include/exclude properties on mask, implement data_valid