License issues with gt4py optional imports

[nbren12]

Apparently, wrapping an import with a try-catch might not be enough to avoid licensing issues with GPL. See this comment: Unidata/netcdf4-python#1073 (comment).

This library uses gt4py in the same manner:
https://github.com/VulcanClimateModeling/fv3gfs-util/blob/52cab8d1ed0bb8ce17cf2695d3075b8484cad218/fv3gfs/util/utils.py#L6

Is this business of mocking out the upstream library whenever we use it really kosher?

[nbren12]

cc @mcgibbon @ofuhrer