Shape Tests

[ilan-gold]

This PR adds some tests around the shapes of implemented methods. Going to have a second pass over this because it's a bit to keep track of. This brings back up what exactly ybar is...stil not sure.

[picciama]

@ilan-gold do we still need this? In my opinion, we can check all of this during model initialisation, that means during creation of an InputDataGLM object. Everything downstream is using properties from the model class so there everything should be correct.
I.e. x, design_loc/scale, constraints_loc/scale, size_factors, theta_location and theta_scale are to be checked once.

[ilan-gold]

Are you saying we can reliably close this? I understand what you're saying, which makes sense, so we should add a check to the __init__ method? Or you're saying things would break loudly if this were wrong?