ENH: apply to other planets?

[jklenzing]

Is your feature request related to a problem? Please describe.
Current software assumes Earth as the default planet for orbital calculations. This could be expanded to other planets.

Describe the solution you'd like
Add other mass / radius info for potential planets. May need to update output since geospacepy assumes earth.

Describe alternatives you've considered
hard-wire to earth. It's possible that we can only get to ECI-like coords from the system, which may not be as useful.

Additional context
from review of 0.3.0 release candidate.