feat: Configurable optimizer tolerance for termination

[kratsg]

Pull Request Description

This PR fully resolves #929.

/cc @alexander-held

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 configurable optimizer tolerance for minimization termination
   - NB: SciPy defaults to None, iminuit defaults to 0.1
* Add tests to ensure the API for SciPy/iminuit

[codecov]

Codecov Report

Merging #1184 (b919fb4) into master (2010b6d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1184   +/-   ##
=======================================
  Coverage   97.43%   97.44%           
=======================================
  Files          63       63           
  Lines        3669     3674    +5     
  Branches      522      522           
=======================================
+ Hits         3575     3580    +5     
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
unittests	97.44% <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/optimize/opt_minuit.py	100.00% <100.00%> (ø)
src/pyhf/optimize/opt_scipy.py	100.00% <100.00%> (ø)

---

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 2010b6d...b919fb4. Read the comment docs.

[matthewfeickert]

LGTM. Thanks @kratsg