feat: Test statistic specified by string instead of using qtilde kwarg

[kratsg]

Description

This replaces qtilde=True and qtilde=False with test_stat="qtilde" and test_stat="q" everywhere. At the same time, we keep around qtilde for backwards-compatibility and mark it as deprecated in v0.6.0 . Likely we will remove this in v0.6.1.

This is more work towards getting PR #520 in.

ReadTheDocs build: https://pyhf.readthedocs.io/en/feat-flexibleteststatistic/api.html#inference

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• Tests are passing
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* Deprecate qtilde in favor of test_stat keyword argument
* Test statistic now specified as a string, instead of relying just on qtilde=True/False

[codecov]

Codecov Report

Merging #1231 (738f9b6) into master (f4df777) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1231   +/-   ##
=======================================
  Coverage   97.45%   97.45%           
=======================================
  Files          63       63           
  Lines        3688     3700   +12     
  Branches      524      524           
=======================================
+ Hits         3594     3606   +12     
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
unittests	97.45% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/pyhf/infer/__init__.py	100.00% <ø> (ø)
src/pyhf/infer/intervals.py	100.00% <ø> (ø)
src/pyhf/cli/infer.py	97.61% <100.00%> (-0.03%)	⬇️
src/pyhf/exceptions/__init__.py	100.00% <100.00%> (ø)
src/pyhf/infer/calculators.py	100.00% <100.00%> (ø)
src/pyhf/infer/utils.py	100.00% <100.00%> (ø)

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[matthewfeickert]

@kratsg I really like this PR overall a lot! Thanks for it and nice idea. :) I do have a suggestion on the API though, so let me know your thoughts.

[matthewfeickert]

We should also grab the notebooks and README in this PR as well

f$ git grep "qtilde=True"
README.rst:   >>> CLs_obs, CLs_exp = pyhf.infer.hypotest(test_mu, data, model, qtilde=True, return_expected=True)
README.rst:   >>> CLs_obs, CLs_exp = pyhf.infer.hypotest(test_mu, data, model, qtilde=True, return_expected=True)
README.rst:       pyhf.infer.hypotest(test_poi, data, model, qtilde=True, return_expected_set=True)
README.rst:       pyhf.infer.hypotest(test_poi, data, model, qtilde=True, return_expected_set=True)
docs/examples/notebooks/hello-world.ipynb:    "    test_mu, data, model, qtilde=True, return_expected=True\n",
docs/examples/notebooks/hello-world.ipynb:    "    test_mu, data, model, qtilde=True, return_tail_probs=True\n",
docs/examples/notebooks/hello-world.ipynb:    "    test_mu, data, model, qtilde=True, return_expected_set=True\n",
docs/examples/notebooks/toys.ipynb:    "    qtilde=True,\n",
src/pyhf/infer/calculators.py:            ``qtilde=True`` has been deprecated in favor of ``test_stat="qtilde"``.
src/pyhf/infer/calculators.py:            ``qtilde=True`` has been deprecated in favor of ``test_stat="qtilde"``.

[matthewfeickert]

Thanks for the beautiful PR @kratsg. I've made my final changes to also bring this to the README and notebooks. Let me know your thoughts but looks good on my side.

[matthewfeickert]

cc @alexander-held RE: a heads up on API shifts in v0.6.0.