feat: minimizer configuration for fit.fit

[alexander-held]

Adding minimizer configuration options to cabinetry.fit: strategy, maxiter and tolerance allow configuring the Minuit strategy, maximum number of iterations and tolerance for Migrad. This extends the fit API functionality, similarly to #320 and #321.

When fitting with custom_fit=True, a ValueError is now raised if the fit fails.

Since the pyhf fit API does not support strategy=None currently (see scikit-hep/pyhf#1785), the strategy kwarg is only propagated for values other than None. This can be refactored in the future in case this changes.

The integration test changes are due to the default tolerance with custom_fit=True now matching the default when using the pyhf fit API (previously the tolerance was reduced by a factor of ten).

partially addresses #329

* added minimizer configuration options to fit.fit
* new supported keyword arguments are strategy, maxiter and tolerance
* raise ValueError when fits with custom_fit=True fail

[codecov]

Codecov Report

Merging #330 (840397a) into master (fa1b7a2) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #330   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines         1925      1933    +8     
  Branches       310       312    +2     
=========================================
+ Hits          1925      1933    +8     

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

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