store bunit in metadata
added unit conversion to a lot of functions
unit(1) instead of 1
whoa u.one is fast
u.one -> u.dimensionless_unscaled
strip data units if data come in with units (let spectral_cube handle
them).  Relative import in tests.  Use check_endian'd data if it's
checked.
fixed bug: _get_filled_data should *not* return a Quantity, whereas
filled_data should
remove unnecessary has_unit kw
u.Quantity(..., unit, copy=False)
refactor moment tests to use integral for moment0
Tick typo
Typo: `data_filled` -> `filled_data`
integral should be K km/s
Merge pull request #53 from keflavich/united

Store data units
Merge pull request #68 from ChrisBeaumont/masks

Move mask code into dedicated module
Documentation and small bugfixes