This takes over from PR #1927
see comments there
move check for at least two groups to use groupunique
ENH: allow group_order to select a subsets of labels in the data.
This was a bug and is now a feature.
The way the labels were initialized, the missing labels were assigned integer label 0.
Better error handling in MultiComparison
Removed file that was added accidentally
Added unit test
ENH: MultiComparison check input in group_order, allow subsets
Coverage increased (+0.01%) when pulling 4f5b4a8 on josef-pkt:bug_multicomp_1927_rebased into 2d6b4b4 on statsmodels:master.