DT/RF: Ensure cat features are recorded per group

JIRA: MADLIB-1254

If tree_train/forest_train is run with grouping enabled and if one of
the groups has a categorical feature with just single level, then the
categorical feature is eliminated for that group. If other groups retain
that feature, then we end up with incorrect "bins" data structure built
as part of DT.

This commit fixes this issue by recording the categorical features
present in each group separately.

Closes apache#295