feat: parameter customization for maximum likelihood fits

[alexander-held]

This adds customization to maximum likelihood fits via fit.fit:

• initial parameter values via init_pars,
• setting parameters to constant via fix_pars,
• parameter bounds via par_bounds.

The names of the kwargs generally follow the pyhf naming, however fix_pars currently differs from the currently used fixed_params used in pyhf (scikit-hep/pyhf#1757). A harmonization in pyhf should probably be propagated to cabinetry as well in case it happens.

This change revealed a small type inconsistency in pyhf-provided default parameter bounds, which is being followed up in scikit-hep/pyhf#1755.

* added parameter customization options to fit.fit API
* new supported keyword arguments are init_pars, fix_pars, and par_bounds

[codecov]

Codecov Report

Merging #320 (2198edf) into master (c1bd802) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #320   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           23        23           
  Lines         1880      1880           
  Branches       304       304           
=========================================
  Hits          1880      1880           

Impacted Files	Coverage Δ
src/cabinetry/fit/__init__.py	100.00% <100.00%> (ø)

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