Processed variables now get spatial variables automatically

[brosaplanella]

Description

Processed variables now automatically take the names of the domains to plot automatically from spatial variables. This was to enable the plotting of 2D variables in arbitrary domains. Note that it required adding a geometry variable to the model as at the moment there is no way to get the geometry from a processed model, even though geometry is a key part of the model. Probably I didn't do it in the most efficient way so suggestions are welcome.

Fixes #3200

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 (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s 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

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f0cc984) 99.71% compared to head (cde00f4) 99.71%.
Report is 21 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3234   +/-   ##
========================================
  Coverage    99.71%   99.71%           
========================================
  Files          248      248           
  Lines        18826    18829    +3     
========================================
+ Hits         18772    18775    +3     
  Misses          54       54           

Files Changed	Coverage Δ
pybamm/discretisations/discretisation.py	99.78% <100.00%> (+<0.01%)	⬆️
pybamm/meshes/meshes.py	98.19% <100.00%> (+0.12%)	⬆️
pybamm/models/base_model.py	99.30% <100.00%> (+<0.01%)	⬆️
pybamm/simulation.py	100.00% <100.00%> (ø)
pybamm/solvers/processed_variable.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[valentinsulzer]

Looks good, just one comment about the public/private API

[valentinsulzer]

I don't think this should have a public setter class