Increase consistency of extending the repetition axis for analytic contrast modules

Problem:

The broadcasting behaviour is not uniformly defined for all contrast modules, which makes it hard to stack modules. Now the broadcasting is implicitly handled depending on the input shape

Proposal:

Add a Variable to all modules that is explictly set on instantiation, to define a controlflow of in call either expanding the axis by the self.expansion_factor or otherwise asserting the input already having the correct number of repetions matching the self.expansion_factor attribute.