#1406 fixed bug with positive electrode ohmic losses

[brosaplanella]

Description

Fixed the sign in the definition of positive electrode overpotential.

Fixes #1406

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: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three 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

Merging #1407 (687dff4) into develop (2c223ba) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1407   +/-   ##
========================================
  Coverage    98.22%   98.22%           
========================================
  Files          278      278           
  Lines        15806    15806           
========================================
  Hits         15526    15526           
  Misses         280      280           

Impacted Files	Coverage Δ
...ybamm/models/submodels/electrode/base_electrode.py	100.00% <100.00%> (ø)

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[valentinsulzer]

Good catch, thanks @brosaplanella