Changed current_sigmoid_ocp to be valid for both electrodes

[DrSOKane]

Description

The existing version of current_sigmoid_ocp is only valid for the negative electrode and gives the wrong result for hysteresis in the positive electrode. Fortunately, adding an extra four lines of code fixes this.

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.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (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
• The documentation builds: $ python run-tests.py --doctest

You can run unit and doctests together at once, using $ python run-tests.py --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

Base: 99.69% // Head: 99.69% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (e00c3c4) compared to base (4f03df4).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2719   +/-   ##
========================================
  Coverage    99.69%   99.69%           
========================================
  Files          271      271           
  Lines        19616    19620    +4     
========================================
+ Hits         19556    19560    +4     
  Misses          60       60           

Impacted Files	Coverage Δ
...face/open_circuit_potential/current_sigmoid_ocp.py	100.00% <100.00%> (ø)

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[valentinsulzer]

Would it be clearer to define lithiation_current with opposite signs for positive and negative electrodes?

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