identify coordinate systems in Orbit and Ephem objects

[mommermi]

clarify the coordinate systems used in Orbit and Ephem objects

[mommermi]

I added some links to the from_horizons docstring to clarify the default reference system for both orbital elements and ephemerides, as well as a short explanation on how to change them. The same will be done for from_mpc and other query wrappers as soon as they are implemented. I also added links that point to field name definitions for properties that are queried.

I chose this approach over explicitly adding this information to the query results as that approach would lead to a large overhead (e.g., adding a refsys/refplane column to the corresponding object would only bloat the underlying table object).

changes are commited here: 4d616d1

[mkelley]

If we used SkyCoord, the reference system would be explicitly carried. I hope we can move to that some day. Perhaps the Ephem object should have a reference system attribute. QTables have a way to carry metadata that we are not taking advantage of.