ENH: custom WGS model import

[jklenzing]

Is your feature request related to a problem? Please describe.
WGS model is hardwired for instrument

Describe the solution you'd like
Add custom kwarg to allow users to select WGS84 or WGS72.

Describe alternatives you've considered
pain and confusion

Additional context
Need to deprecate default for missions_sgp4, new default will be WGS84.