Handle partially fixed parameters in model_utils API

[alexander-held]

The support for partially fixed parameters (such as staterror with zeros in some bins) in pyhf 0.7 can result in scenarios where the behavior of suggested_fixed_as_bool is undefined, see scikit-hep/pyhf#1944.

This currently breaks model_utils.prefit_uncertainties in cabinetry (and also affects model_utils.unconstrained_parameter_count). Working around this should be possible by iterating bin by bin for affected parameters and using the suggested_fixed API instead.

[alexander-held]

model_utils.prefit_uncertainties can be fixed by using the suggested_fixed API. model_utils.unconstrained_parameter_count is not affected by constrained parameters, but needs in principle to be able to handle shapefactor modifiers that are partially fixed. This is not currently supported by pyhf on a workspace specification level (scikit-hep/pyhf#2053), but it would be good to already implement this in cabinetry such that it automatically works if support arrives in pyhf.