Issue 1456 concentration

[valentinsulzer]

Description

Makes "porosity times concentration" the variable in the full diffusion model. This could also be changed in the other diffusion models, but I don't need it right now and am lazy. At some point we will need to change the diffusion model for the SPM to account for the effect of porosity changes on average concentration.

Fixes #1456

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 #1476 (dfe802d) into develop (a31e209) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1476   +/-   ##
========================================
  Coverage    98.37%   98.37%           
========================================
  Files          282      282           
  Lines        16602    16626   +24     
========================================
+ Hits         16332    16356   +24     
  Misses         270      270           

Impacted Files	Coverage Δ
pybamm/models/base_model.py	99.19% <ø> (-0.01%)	⬇️
pybamm/expression_tree/binary_operators.py	99.66% <100.00%> (+<0.01%)	⬆️
pybamm/models/standard_variables.py	100.00% <100.00%> (ø)
...lectrolyte_diffusion/base_electrolyte_diffusion.py	100.00% <100.00%> (ø)
...ls/electrolyte_diffusion/constant_concentration.py	100.00% <100.00%> (ø)
.../submodels/electrolyte_diffusion/full_diffusion.py	100.00% <100.00%> (ø)

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[rtimms]

thanks @tinosulzer !