Create MomentMap class to encompass the moment map with WCS information, and make it easy to get an HDU object using the ``hdu`` attribute.
Create MomentMap class to encompass the moment map with WCS information, and make it easy to get an HDU object using the ``hdu`` attribute.

radio-astro-tools/spectral-cube,low-sky/spectral-cube,e-koch/spectral-cube,jzuhone/spectral-cube,keflavich/spectral-cube