Allow particle size to be a tuple and fix MPM half-cell

[rtimms]

Description

"Particle size" can now be a tuple to allow different behaviour in each electrode. MPM now works as a half-cell model, and no longer requires the user to provide distribution parameters for both electrodes.

Also allows SPM(e) to have size distributions so that users can create more customised models (e.g. SPMe with a size distribution in one electrode only)

Fixes #2669

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

Coverage data is based on head (3c30f50) compared to base (a3756de).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2672   +/-   ##
========================================
  Coverage    99.69%   99.69%           
========================================
  Files          271      271           
  Lines        19571    19586   +15     
========================================
+ Hits         19511    19526   +15     
  Misses          60       60           

Impacted Files	Coverage Δ
pybamm/geometry/battery_geometry.py	100.00% <100.00%> (ø)
...m/models/full_battery_models/base_battery_model.py	99.78% <100.00%> (ø)
...bamm/models/full_battery_models/lithium_ion/mpm.py	100.00% <100.00%> (ø)
.../submodels/active_material/base_active_material.py	100.00% <100.00%> (ø)
...ybamm/models/submodels/interface/base_interface.py	100.00% <100.00%> (ø)
...dels/submodels/interface/kinetics/base_kinetics.py	100.00% <100.00%> (ø)
...face/open_circuit_potential/current_sigmoid_ocp.py	100.00% <100.00%> (ø)
...els/interface/open_circuit_potential/single_ocp.py	100.00% <100.00%> (ø)
pybamm/models/submodels/particle/base_particle.py	100.00% <100.00%> (ø)
pybamm/parameters/size_distribution_parameters.py	100.00% <100.00%> (ø)
... and 46 more

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[valentinsulzer]

Looks good to me. How much effort is fixing the coverage?