make a new Mask with appropriate wcs
add with_spectral_unit for masks that have associated wcs attributes
finish with_spectral_unit implementation for the Masks.  Extracted the
docstring to a global
update with_spectral_unit in spectral_cube to apply to the mask too
implemented a test and fixed bugs so that it works
found more corners that needed testing and fixing
buff up a test a little
tests for spectral_cube.with_spectral_unit
ridiculous typo
a little extra doc refactor
remove unnecessary import
a change I had previously made was lost: the existence of this commit
opens the worrying possibility that other changes were lost
added WCS utility to check for equality between WCSes, incorporated into
masks, and added a relevant test
masks can't be data arrays, they have to be masks
add tests for other mask types
add with_spectral_wcs tests for other mask types
Merge pull request #88 from keflavich/issue88

``with_spectral_unit`` does not change mask WCS