ENH: Add georeference module, basic tests

[mgrover1]

Add some georeference helper functions, first steps to georeferencing the xradar object

[kmuehlbauer]

Thanks @mgrover1 for setting this up. :+1:

Just some questions, comments and thoughts:

• Is there a specific need to have geographic_to_cartesian_aeqd in the codebase?
• It would be great to parameterize earth radius as well as k.
• I'd suggest to use metric SI units (meters in this example), as the model keeps the ranges in meters.
• We can use site-coordinates (lat, lon, alt) to calculate final bin aeqd coordinates.
• We might use latitude dependend earth radius for the calculations.
• We might return a projstring with the used AEQD projection.

Basically in wradlib we are using the same equations derived Doviak. We might utilize the existing wradlib function which adds the x, y, z and groundrange coordinate DataArrays.

[mgrover1]

Yeah - on second thought, I like that idea of operating at the dataset level instead of the numpy level. This is using the current functions/methods we have in Py-ART, but adapting it to work at the dataset level is a better approach.

I will make those changes, and ping you when it is ready for review.

[kmuehlbauer]

Ready to go, @mgrover1 ?

[mgrover1]

Sure :)

[kmuehlbauer]

Thanks @mgrover1, this is going to be fun to add to the current examples.