feat: Propagate model parameter names to optimizers

[kratsg]

Pull Request Description

Resolves #1099. This passes through parameter names to the optimizers, both scipy and minuit. This is done by adding ModelConfig.par_names() which returns the parameter names in a format that has yet to be agreed upon (for indexing n-binned parameters). This function output is threaded through the optimizer framework via the Optimizer._get_minimizer() function call. However, scipy does not make use of this, so not much changes except to support the updated function signature, while Minuit is able to take advantage of this.

A format string is allowed as an optional keyword argument so we're not locked into a specific format.

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* Propagate model parameter names through to the optimizers (primarily Minuit)
   - Additionally allow a customizable format for the parameter names
* Add par_names method to _ModelConfig
* Add tests for _ModelConfig.par_names

[kratsg]

Currently, this does [mu, uncorr_bkguncert[0], uncorr_bkguncert[1]] -- but we could/should discuss the various naming schema allowed. One could consider allowing the user to choose how the naming schema is done by allowing them to provide a format string.

Customizable via optional fstring kwarg.

[codecov]

Codecov Report

Merging #1536 (01f9beb) into master (a0e2568) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1536   +/-   ##
=======================================
  Coverage   97.65%   97.66%           
=======================================
  Files          63       63           
  Lines        4006     4023   +17     
  Branches      565      571    +6     
=======================================
+ Hits         3912     3929   +17     
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.42% <5.26%> (-0.09%)	⬇️
unittests	97.43% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/pyhf/optimize/opt_scipy.py	100.00% <ø> (ø)
src/pyhf/optimize/mixins.py	100.00% <100.00%> (ø)
src/pyhf/optimize/opt_minuit.py	100.00% <100.00%> (ø)
src/pyhf/pdf.py	96.54% <100.00%> (+0.10%)	⬆️

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[matthewfeickert]

Minor comments (which I'll check now on the formatting and merge if it looks okay), but this all looks good.

[matthewfeickert]

Yeah, as that checks out in the docs build this all LGTM. Thanks @kratsg!