Porosity times concentration

[valentinsulzer]

Description

Update constant concentration model so that "porosity times concentration" is conserved.
This fixes a bug where porosity was changing but concentration was staying constant, which meant that porosity times concentration was changing

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

Base: 99.73% // Head: 99.73% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (22e9d5a) compared to base (bff1f46).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2529   +/-   ##
========================================
  Coverage    99.73%   99.73%           
========================================
  Files          270      270           
  Lines        19360    19373   +13     
========================================
+ Hits         19308    19321   +13     
  Misses          52       52           

Impacted Files	Coverage Δ
pybamm/models/submodels/particle/base_particle.py	100.00% <ø> (ø)
...ttery_models/lithium_ion/base_lithium_ion_model.py	100.00% <100.00%> (ø)
...models/active_material/constant_active_material.py	100.00% <100.00%> (ø)
.../submodels/active_material/loss_active_material.py	100.00% <100.00%> (ø)
...ls/electrolyte_diffusion/constant_concentration.py	100.00% <100.00%> (ø)
pybamm/solvers/processed_variable.py	100.00% <100.00%> (ø)
pybamm/util.py	100.00% <100.00%> (ø)

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[brosaplanella]

Looks good, just a couple of minor comments.