feat: Allow POI-less models via Workspace.model

[kratsg]

Pull Request Description

No associated issues. Primarily drop the poi_name functionality in pyhf.Workspace.get_measurement as it is not really used and is kind of a bit weird anyway. Instead, allow for users to specify poi_name in pyhf.Workspace.model() in order to override the suggested POI that comes from the measurement they would like to use. This allows a user to use an existing measurement, but override the poi_name.

For example, pyhf.Workspace.model(poi_name=None) will allow for background-only models to be created from pyhf.Workspace which was not something that was allowed in the past.

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

* Remove `poi_name` functionality in `pyhf.Workspace.get_measurement`
* Allow for `poi_name` to be optionally set in `pyhf.Workspace.model` to override suggested POI
* Add tests for setting poi_name through `pyhf.Workspace.model`
* Provide intersphinx for jsonpatch docs

[codecov]

Codecov Report

Merging #1636 (843a11b) into master (6f4ca1a) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1636      +/-   ##
==========================================
+ Coverage   97.99%   98.03%   +0.04%     
==========================================
  Files          63       63              
  Lines        4132     4128       -4     
  Branches      567      566       -1     
==========================================
- Hits         4049     4047       -2     
+ Misses         49       48       -1     
+ Partials       34       33       -1     

Flag	Coverage Δ
contrib	25.00% <28.57%> (+0.02%)	⬆️
unittests	97.81% <100.00%> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
src/pyhf/workspace.py	100.00% <100.00%> (ø)
src/pyhf/pdf.py	97.69% <0.00%> (+0.65%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6f4ca1a...843a11b. Read the comment docs.

[matthewfeickert]

Relevant RTD build: https://pyhf--1636.org.readthedocs.build/en/1636/_generated/pyhf.workspace.Workspace.html

[matthewfeickert]

@kratsg This is nice. :)

I have two render changes and then a docstring length suggestion, but I like this.

[alexander-held]

related: #1602

[matthewfeickert]

LGTM. 👍 Thanks @kratsg!