ENH: Add GEMINI model object

[jklenzing]

Is your feature request related to a problem? Please describe.
The GEMINI model (Geospace Environment Model of Ion-Neutral Interactions) is a high-resolution three-dimensional ionospheric fluid-electrodynamic model. Comparison to satellite data, rocket campaigns, and radar campaigns would benefit from a pysatModelUtils interface.

Describe the solution you'd like
Add a model object to interface with output from the GEMINI ionospheric model. Include the capability to only load a slice of the model, since the output is very large.

Additional context
Current GEMINI code is here: https://github.com/gemini3d/GEMINI
Upgrade to pyGEMINI (including new file output) in progress.