Improvements to `keplerlib.py` and `elementslib.py`

I’d like to start a discussion of what things in keplerlib.py and elementslib.py can be improved, and also putting them in priority order so I can work on the more important things first. Here are some ideas I’ve had:

• Should all of the math functions in elementslib have one code path that can handle floats or arrays instead of one path for arrays and one path for floats? Possibly a decorator could convert floats into arrays and then back to floats so only the array branch would be necessary
• Should the KeplerOrbit and OsculatingElements objects use au^3/d^2 to store GM internally to match the rest of Skyfield instead of km^3/s^2? I think the only place this would effect users is that OsculatingElements has a keyword argument mu_km_s. How should this be handled?
• Should the user be expected to specify a value for GM whenever it is necessary rather than saving values internally? If so how should this be handled for backwards compatibility?
• I need to refamiliarize myself with what is happening in PR #340 so it can be either merged or closed
• After the signature for KeplerOrbit objects is finalized it can be specified in the API reference section.
• revise the documentation for osculating orbital elements so it better matches the rest of the documentation both in tone and in the formatting of the examples

[JoshPaterson]

Is it clear enough what this would look like that you could try a quick draft PR, or should I suggest a rough draft of what the API would look like, to save you a round-trip if you are imagining something a bit different?

Yes, this feels clear enough to make a draft PR.

any further dimensions should work seamlessly, as long as both time and coordinates are extended to the same number of dimensions.

Interesting! I think KeplerOrbit objects are similar in that once keplerlib.propagate is vectorized the KeplerOrbit doesn’t need to change, the arrays are just passed right through.

[Bernmeister]

@brandon-rhodes Am crafting said script (it is similar to that in #449 but taking into account your suggestions with regards to dataframe loading). I’ll post the script in a new ticket so this doesn’t get muddied.