Debug syst_toys to run fixed_params toys

Few changes to be able to run fixed_params toys using syst_toys.py. Would be great if someone else could check that this works fine for them too (did it work before?). I attach the configuration and previous fit results I'm using for an easy check but if someone has other examples that would be better.

All the changes are restricted to syst_toys.py itself and systematics.py so I think other parts of the framework should be unaffected. Would be better to cross-check ofc. I'm only concerned about the other type of toys for systematics since they might be affected by the changes. Would be great if anyone could check.