BUG: Instrument registration does not work

[jklenzing]

Describe the bug
If I try to register the instruments in pysat, the registry is updated accordingly. However, pysat will search in the main package for the instruments rather than loading them from the registry.

To Reproduce

import pysat
import pysatMissions
pysat.utils.registry.register_by_module(pysatMissions.instruments)
sgp4 = pysat.Instrument('pysat', 'sgp4')

produces

ModuleNotFoundError: No module named 'pysat.instruments.pysat_sgp4'

Presumably this is because the platform name is pysat.

Expected behavior
Should load from pysatMissions.

Additional context
This could be solved here by renaming the platform or in the main package by adjusting how the code works. Tagging @rstoneback.

[rstoneback]

The ‘pysat’ platform is reserved for instrument support files within pysat itself. The platform name for pysat_sgp4 will need to be updated to something else.

[jklenzing]

Possibilities for a new platform:

• orbprop
• orbital
• propagator
• missions (AGB suggestion)

[jklenzing]

Closing with merge of #52