Refactor to put categorical models in subclasses
Fix float division problem for py3
Added scale field to summary
Changed wording in independence covstruct summary
Work on doc strings
Allow from_formula to specify time and group by variable name
Edit release notes example to reflect interface change
Merge pull request #1823 from josef-pkt/gee-cat-subclass_1694_rebase

REF: Gee cat subclass 1694 rebased, closes #1694