feat: par_names handles non-scalar modifiers with 1 parameter

[kratsg]

Pull Request Description

Resolves #867.

Should fully resolve the last remaining parts of #867. There are two issues handled:

• par_names did allow for customizable fstring, but this would never get passed through to the underlying optimizers that use it, so it's being dropped as the benefit of configurability is outweighed by the extra work on the user to maintain and bookkeep
• par_names now correctly uses the fact that parameter sets which are non-scalar but only have one parameter (e.g. channel of 1 bin) will still be named with the index {parameter}[0] rather than just {parameter} to clarify that it's a non-scalar parameter explicitly (but also somewhat implicitly).

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* change par_names API to drop fstring configurability
* modify par_names to handle non-scalar parameters in 1-bin channels

[codecov]

Codecov Report

Merging #1560 (c203b14) into master (bd1d571) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1560   +/-   ##
=======================================
  Coverage   97.70%   97.70%           
=======================================
  Files          63       63           
  Lines        4050     4050           
  Branches      576      576           
=======================================
  Hits         3957     3957           
  Misses         54       54           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.40% <33.33%> (ø)
unittests	97.48% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/pyhf/pdf.py	96.54% <100.00%> (ø)

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[kratsg]

As the changes are pretty minor (especially given the original functionality was ok in the originating PR), I'm going to merge this in so @matthewfeickert can focus on getting the release out.

[matthewfeickert]

@kratsg as this is API breaking do we know if this is problematic for cabinetry at all? cc @alexander-held.

I'm optimistic that is isn't a problem, given that you only had to add a test and not remove any.

[kratsg]

not API breaking as this wasn't in the v0.6.2 API. It's just changes to the API on the main branch.

[matthewfeickert]

Ah that's fantastic. Thanks for clarifying that.