Mapping of CASA's coordinate frame to astropy's coordinate frame.

[sushobhana]

https://casaguides.nrao.edu/index.php/Coordinate_Frames . This link contains CASA coordinate frames . We need to map them into astropy's coordinate frames. Apart from 'j2000', 'icrs', 'galactic', 'supergal', 'ecliptic', 'b1950', I am not able to identify others.

[keflavich]

We can safely ignore the other ones for now. They're not very likely frames to use when defining regions, and I don't know yet whether astropy is capable of supporting transformations between several of them. We should raise NotImplementedErrors when encountering the various terrestrial frames.