Merge pull request #37 from ChrisBeaumont/tests

New tests and some bugfixes
Use np.isfinite as a mask function
Refactored masks to enable non-array masks, and implemented function masks
Fix bugs in masking
Added tests for masking
Added tests for functional mask
Compare WCS using to_header()
Fix test failures
Added tests for flattened
Removed shape from function mask since it doesn't make sense there
Refactor so that include, exclude don't require data and WCS, and flattened and filled don't require WCS
Remove unnecessary copy()
Added TODO about efficiently in SpectralCubeMask
Separate out implementations of LazyMask and FunctionalMask
Fix test failures
Implement LazyMask slicing
Added test for slice_wcs
Implemented comments from @keflavich
Default slices to () instead of None to avoid adding a dimension
Added docstring and step checking for slice_wcs