Fix half cell bug

[brosaplanella]

Description

Fixes the bug with the potential on half-cell models

Fixes #1132

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 #1133 into develop will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1133      +/-   ##
===========================================
+ Coverage    97.90%   97.92%   +0.02%     
===========================================
  Files          246      247       +1     
  Lines        13572    13913     +341     
===========================================
+ Hits         13287    13625     +338     
- Misses         285      288       +3     

Impacted Files	Coverage Δ
..._battery_models/lithium_ion/basic_dfn_half_cell.py	99.21% <100.00%> (+0.01%)	⬆️
pybamm/solvers/casadi_solver.py	100.00% <0.00%> (ø)
pybamm/models/standard_variables.py	100.00% <0.00%> (ø)
pybamm/meshes/one_dimensional_submeshes.py	100.00% <0.00%> (ø)
pybamm/models/submodels/particle/base_particle.py	100.00% <0.00%> (ø)
...bamm/models/full_battery_models/lithium_ion/dfn.py	100.00% <0.00%> (ø)
...bamm/models/full_battery_models/lithium_ion/spm.py	100.00% <0.00%> (ø)
...amm/models/full_battery_models/lithium_ion/spme.py	100.00% <0.00%> (ø)
...odels/submodels/particle/fickian_many_particles.py	100.00% <0.00%> (ø)
...dels/submodels/particle/fickian_single_particle.py	100.00% <0.00%> (ø)
... and 6 more

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[rtimms]

thanks @ferranbrosa ! could you update the changelog too

[brosaplanella]

thanks @ferranbrosa ! could you update the changelog too

Done! I was about to do so, you are too efficient hahaha

[rtimms]

haha always waiting for the next PR to turn up!