StatsModels: Make `group_by` mandatory

[adelavega]

Given that for simple models (i.e. contrasts at run level, and dummy contrasts at subject and dataset level) GroupBy must be set, I can't imagine a situation where letting pybids set a default GroupBy would be a good idea.

We have also gone in the direction of explicit > implicit, so I would vote to make this argument mandatory.

[adelavega]

This is mandatory in the spec, so if not this is a bug