fix: Add guards against shared shapesys paramsets

[kratsg]

Pull Request Description

• Resolves Correlated shapesys across channels #1908
• Resolves Shapesys modifier correlated between samples #1967
• Resolves shapesys regressions blocking 0.7.0 #1973
• Amends PR feat: Add setup for custom modifiers #1625

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• 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

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For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add checks to determine if modifiers should be shared across channels when
  the modifier is being built. This guards against accidental correlations across
  channels. e.g. shared shapesys paramsets.
   - Amends PR #1625
* Add tests for raised pyhf.exceptions.InvalidModel in the event that a shared shapesys
  paramset is introduced in the model spec.

[codecov]

Codecov Report

Base: 98.25% // Head: 98.26% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (39fe675) compared to base (6bbd9ff).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1977   +/-   ##
=======================================
  Coverage   98.25%   98.26%           
=======================================
  Files          68       68           
  Lines        4420     4430   +10     
  Branches      727      728    +1     
=======================================
+ Hits         4343     4353   +10     
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	27.04% <43.75%> (+0.09%)	⬆️
doctest	60.88% <87.50%> (+0.04%)	⬆️
unittests-3.10	96.18% <100.00%> (+<0.01%)	⬆️
unittests-3.7	96.16% <100.00%> (+<0.01%)	⬆️
unittests-3.8	96.20% <100.00%> (+<0.01%)	⬆️
unittests-3.9	96.23% <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/modifiers/histosys.py	100.00% <100.00%> (ø)
src/pyhf/modifiers/lumi.py	100.00% <100.00%> (ø)
src/pyhf/modifiers/normfactor.py	100.00% <100.00%> (ø)
src/pyhf/modifiers/normsys.py	100.00% <100.00%> (ø)
src/pyhf/modifiers/shapefactor.py	100.00% <100.00%> (ø)
src/pyhf/modifiers/shapesys.py	100.00% <100.00%> (ø)
src/pyhf/modifiers/staterror.py	98.05% <100.00%> (+0.01%)	⬆️
src/pyhf/pdf.py	98.02% <100.00%> (+0.01%)	⬆️

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[matthewfeickert]

LGTM. 👍 Thanks @kratsg!

@alexander-held both of the example problematic scripts in your excellent Issues #1908 and #1967 now properly raise pyhf.exceptions.InvalidModel. 👍