Move GEO boundary calculations to Pyresample

[pnuu]

Feature Request

There is a multipurpose function satpy.readers.utils.get_geostationary_bounding_box() that should be in pyresample.boundary module.

Is your feature request related to a problem? Please describe.
I noticed this via pytroll/pyresample#228 when working on pytroll/trollflow2#67 . The other boundary calculations are located in Pyresample, so that would be a proper location for it.

Describe the solution you'd like
Move satpy.readers.utils.get_geostationary_bounding_box() and the associated helper functions to pyresample.boundary module.

Describe any changes to existing user workflow
The import of get_geostationary_bounding_box in trollflow2.plugins needs to be updated, along with the import added to satpy where the function is used.

[pnuu]

There seems also to be an issue with this functionality with GOES-16/17, which define the area using ellps: GRS80. The functions used in the calculations expect to have a/b/h available in the proj_dict.

[pnuu]

The functionality already seems to be in Pyresample! See: https://github.com/pytroll/pyresample/blob/master/pyresample/geometry.py#L1948

[pnuu]

Oh, and the a/b/h issue I mentioned above is also fixed there: https://github.com/pytroll/pyresample/blob/master/pyresample/geometry.py#L1929

[djhoese]

Yes, this was fixed in pyresample because newer versions of pyproj/PROJ will replace a/b combinations with the associated ellipsoid/datum if they can figure it out.

We'll need to update these to not use proj_dict and look at attributes in the CRS object in the future but that's a separate issue.