Remove useless fit runcard namespaces

Created by: scarlehoff

In particular the fitting::whatever is no longer used by n3fit and should be removed as it introduces the chance for many different errors (see #1129)

Together with that, some other common errors can be checked (again see #1129).

The problem when doing this is that while n3fit no longer depends on fitting, validphys might rely on it being present at some place within the fit runcard, in order not to break backwards compatibility those cases should be caught.

Other points that could benefit of some changes are things like the positivity dictionary (for instance, the poslambda parameter should rather be the maxlambda). These might require other changes in vp but it is worth to keep in mind.

Assigning myself but I'm more than happy to leave the task to someone else :P