minor fixes and convenience functions
remove set_mask
rename some get functions
change LazyMask __init__ spec
fix lazy mask test
ndim, size
revert changes to header checking
split get_mask and get_mask_array
fix IO for LazyMask stuff
laziness unlazified lazily (this is meaningless)
made lazy instantiable from data&wcs or cube
fix tests
fix more tests
tests are passing again
Merge pull request #45 from keflavich/minor_fixes

Minor fixes
faster nanmedian with bottleneck