ENH: Common metadata checks

[jklenzing]

Is your feature request related to a problem? Please describe.
Maintainability could be improved by defining metadata in a single location. This could be imported across multiple instruments.

Describe the solution you'd like
A single metadata check somewhere. Either add as part of functions (eg, geospacepy methods) or have a single database that checks variable names.

Describe alternatives you've considered
Keep things as they are.

Additional context
Pings from codeclimate about file length and repeated code statements. missions_ephem is deprecated and will be removed, but new instrument modules may take its place.