Changed covstruct argument to cov_struct
Added a bit to the release announcement for GEE
Improved one line of linear algebra
Remove local test code
Update tests
Changed covstruct to cov_struct
Doc string fix
Added diagnostic plot for covariance structure
Rename the plot function
Refactored nominal and ordinal code, added plot function for ordinal
Merge pull request #1553 from josef-pkt/gee_maint_1528_rebased

Gee maint 1528 rebased, closes PR #1528