Make per-group test fit only one model

[Hoeze]

See #35, this is currently done by fitting separate models per group.

[davidsebfischer]

Note that this only makes sense for Wald test. This does not work for LRT, T-test and Wilcoxon test, which is why this is only a speed-up enhancement but not a critical fix.

[davidsebfischer]

This would be in ./test/base.py:_Partition.wald()