feat: calculator API

[lukasheinrich]

Description

Introduces the "calculator API" in order to allow to run inference in various modes (toybased, asymptotics, etc).

This might be quite a big PR, we can consider splitting it up into various small PRs and rebasing along the way.

Demo notebook:

https://github.com/scikit-hep/pyhf/blob/calculator_api/docs/examples/notebooks/UsingCalculators.ipynb

Sub-changes:

• change qmu to return scalar instead of size-1-tensor feat: change qmu API and remove test_stat return #712
• rearrange asymptotic CLs calculation refactor: rearrange asymptotics #745
• introduce AsymptoticCalculator feat: Add AsymptoticCalculator #750
• introduce ToyCalculator feat: Add toy calculator, empirical distribution, and toy example notebook #790

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

[codecov]

Codecov Report

Merging #709 into master will decrease coverage by 0.91%.
The diff coverage is 72.38%.

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
- Coverage   94.86%   93.94%   -0.92%     
==========================================
  Files          52       54       +2     
  Lines        2765     2826      +61     
  Branches      393      395       +2     
==========================================
+ Hits         2623     2655      +32     
- Misses         97      125      +28     
- Partials       45       46       +1     

Flag	Coverage Δ
#unittests	93.94% <72.38%> (-0.92%)	⬇️

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Impacted Files	Coverage Δ
src/pyhf/infer/toybased.py	26.31% <26.31%> (ø)
src/pyhf/infer/__init__.py	97.05% <94.44%> (-2.95%)	⬇️
src/pyhf/infer/asymptotics.py	100.00% <100.00%> (ø)
src/pyhf/infer/test_statistics.py	100.00% <100.00%> (ø)
src/pyhf/infer/utils.py	100.00% <100.00%> (ø)

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[lukasheinrich]

rebased to #712

[kratsg]

#1258 adds in the last piece from this PR so I will close this in favor of following up there.