Allow users to initialize layers with SkyCoord instead of Table

[astrofrog]

Astropy allows users to construct arrays of SkyCoord objects which can contain positions on the sky and optionally distances. Essentially this provides absolute 2D or 3D coordinates. We should allow the data layer to be initialized with this and automatically generate a table internally and set all the settings correctly (e.g. reference frame, changing altitude type to distance, and so on).