feat: Allow readxml to demote validation exception to warning

[kratsg]

Pull Request Description

This resolves #1864. Added functionality pyhf.readxml to push the exception down to a warning if needed, so conversion still works for invalid XML workspaces, but will still warn that it is invalid. This also adds it to pyhf xml2json as well.

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

* Add ability to passing validation_as_error=False to readxml.parse to warn when the workspace
is invalid instead of just raising an exception (default is to raise exception).
* Add --validation-as-error/validation-as-warning flag to xml2json command line interface to
switch between the two (default is to raise exception).

[codecov]

Codecov Report

Merging #1865 (1196065) into master (de4ce07) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1865   +/-   ##
=======================================
  Coverage   98.16%   98.17%           
=======================================
  Files          68       68           
  Lines        4315     4322    +7     
  Branches      725      726    +1     
=======================================
+ Hits         4236     4243    +7     
  Misses         46       46           
  Partials       33       33           

Flag	Coverage Δ
contrib	26.39% <0.00%> (-0.05%)	⬇️
doctest	60.73% <36.36%> (-0.06%)	⬇️
unittests-3.10	96.04% <100.00%> (+<0.01%)	⬆️
unittests-3.7	96.02% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/pyhf/cli/rootio.py	92.30% <100.00%> (+0.15%)	⬆️
src/pyhf/readxml.py	94.57% <100.00%> (+0.20%)	⬆️

---

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 de4ce07...1196065. Read the comment docs.

[matthewfeickert]

LGTM. 👍 Thanks for fixing the coverage!