Merge branch 'groups_vs_group' into 'master'

Fixes #33

I am now checking that the ``--group`` and according options are not used w/o ``--sub-task``. 

A test case is yet missing, but will follow soon.

Note that I will need to modify ``bob.bio.gmm`` accordingly, unless someone finds a better and more generic solution to #33 

Accepting this merge will close #33.

See merge request !34