Warn about Model / Fitter incompatibilities

[JasperVanDenBosch]

A mismatched fitter / model combination could probably lead to a more obvious exception, and the model arg could be type annotated for the specific models supported.

For example:

      from numpy.random import rand
      from rsatoolbox.rdm.rdms import RDMs
      from rsatoolbox.model.model import ModelFixed
      from rsatoolbox.model.fitter import fit_optimize
      data = RDMs(rand(3, 2))
      theta = fit_optimize(ModelFixed('F', data), data)

Leads to:

ValueError: attempt to get argmin of an empty sequence