better embedding of mass and potential

[nstarman]

Description

Currently the mass and potential are embedded in a SkyCoord. This does not work very well as any operation that returns a new instance, like reshape means that the attributes need to be reassigned (separately reshaped).

Possible fixes:

• Subclass SkyCoord, with hooks to properly maintain these attributes
• embed in a table (SkyCoord is a valid mixin) ... but can tables be stacked depth-wise?
• Xarray? but how does that deal with SkyCoords?

Checklist

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