instrument_altitude_to_model_pressure as pysat.custom compatible function

[aburrell]

Is your feature request related to a problem? Please describe.
instrument_altitude_to_model_pressure routine does not works as a pysat.custom function for TIEGCM.

Describe the solution you'd like
Change instrument_altitude_to_model_pressure routine works as a pysat.custom function

[aburrell]

Extracting this from PR #36

[rstoneback]

Should I convert all the functions in extract or only instrument_altitude_to_model_pressure?

[aburrell]

Any of the ones that are appropriate. I think the main extract routine isn't an appropriate candidate.

[rstoneback]

ok, I can leave out the extract_modeled_observations one and convert the rest. I'm going to remove the returned keys and convert the xarray dataset input to expect a pysat.Instrument input for the model.

[aburrell]

Hmmm... maybe we should wait then. I am not sure what else will break if we change the inputs.

[aburrell]

How about creating some wrapper functions that use the utilities as pysat functions?