Add _from_json functionality for pybamm.sign

[pipliggins]

Description

_from_json in pybamm.Sign was previously labelled as NotImplemented; #4394 identified this as a mistake as the pybamm.Sign class should be able to be de/serialised. This PR adds that functionality in.

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (d48f8c6) to head (c5f2efe).
Report is 160 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4517      +/-   ##
===========================================
- Coverage    99.42%   99.42%   -0.01%     
===========================================
  Files          301      299       -2     
  Lines        22736    22715      -21     
===========================================
- Hits         22605    22584      -21     
  Misses         131      131              

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[brosaplanella]

Looks good, thanks!