Separate arithmetic from the base NDData class
Separate base NDData tests from NDData arithmetic tests.
Make wcs a property
Docstring clarification
Docstring fix
Update NDData doc pages to reflect new layout
Remove flags from NDData and update documentation
Update CHANGES.rst [ci skip]
Move NDDataArithmetic to separate module
Remove unused imports
Move implementation of uncertainty from NDData to NDDataArithmetic
Move array-based handling of masks into NDDataArithmetic
Remove __array__ methods from bas NDData
Remove setters from most properties in base NDData

Could not change meta without breaking the MetaData tests
Move slicing out of base NDData
Delegate slicing to WCS instead of failing
Require that data have shape attribute and be sliceable
Require uncertainty to have string attribute uncertainty_type
Add NDSlicing to nddata namespace
PEP8 changes