MAINT: pyephem no longer updated

[jklenzing]

Is your feature request related to a problem? Please describe.
pyephem will continue to receive bug updates, but the creators suggest that skyfield is preferred.
see https://rhodesmill.org/pyephem/

Describe the solution you'd like
With the proper enhancements to the sgp4 instrument (#56), this could be dropped.

Describe alternatives you've considered
Update to skyfield? It's more for astronomical objects, not satellites.

Additional context
Originally this was employed to retrieve lat lon alt info. This can be updated via #56

[jklenzing]

Assuming #56 works, I propose we deprecate this in 0.0.3 and drop it in 0.0.4.

[jklenzing]

Deprecation warnings created, version bump

[jklenzing]

Note that OMMBV can be dropped as a requirement when this is removed.