feat: Don't specify a default POI name

[matthewfeickert]

Description

Resolves #2327

To avoid making assumptions for users about model behavior set the default POI name to None.

To avoid making behavior changes for pyhf.simplemodels add the default poi_name to the returned model.

This then requires explicilty setting poi_name="mu" in the tests for all explicity pyhf.pdf.Model constrcution.

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

* Set the default model poi_name to None to avoid making assumptions
  for users.
* To maintain the pyhf.simplemodels users experience add poi_name="mu"
  as a default argument to be passed to the returned pyhf.pdf.Model
  object.
* Add poi_name="mu" to all pyhf.Model construction in the tests.

[matthewfeickert]

If we're going to be changing the default behaivor of pyhf.pdf.Model should we just go all in and make sure that pyhf.simplemodel also requires users to set things? Or should we keep the pyhf.simplemodel experience the same?

[alexander-held]

I think it's fair to automatically set the POI for the simplemodels case. There is no ambiguity there for what else the POI should be.

[matthewfeickert]

We could elevate this to a user accesible argument that defaults to mu and pass that through here, but as you said, the POI for these models is meant to be mu.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.13% 🎉

Comparison is base (10b17dd) 98.14% compared to head (951ad7a) 98.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2328      +/-   ##
==========================================
+ Coverage   98.14%   98.28%   +0.13%     
==========================================
  Files          69       69              
  Lines        4538     4538              
  Branches      803      803              
==========================================
+ Hits         4454     4460       +6     
+ Misses         48       45       -3     
+ Partials       36       33       -3     

Flag	Coverage Δ
contrib	97.86% <100.00%> (ø)
doctest	60.99% <100.00%> (ø)
unittests-3.10	96.29% <100.00%> (ø)
unittests-3.11	96.29% <100.00%> (ø)
unittests-3.8	96.31% <100.00%> (?)
unittests-3.9	96.34% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/pyhf/pdf.py	98.03% <100.00%> (ø)
src/pyhf/simplemodels.py	92.30% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[matthewfeickert]

Relevant RTD build: https://pyhf--2328.org.readthedocs.build/en/2328/_generated/pyhf.simplemodels.uncorrelated_background.html