ENH add GEE to genmod

this is a rebased version of PR #928 by @kshedden
see #928 for discussion
additional cleanups from PR #1215 by @vincentarelbundock 

I will do another rebase and force push just before merging.

The PR doesn't have any example scripts and examples need to be extracted from unittests or simulations.
I haven't tried building the docs yet. (master has currently already many doc build warnings.)
No systematic testing across python and dependency versions yet.

This is a very large enhancement, and followup review and code adjustments are to be expected.