Composite release parameters

[valentinsulzer]

Description

Some changes in preparation for the composite release.

• introduces phase dependency (the "phase_name" in a lot of the variable names, which is empty by default and primary/secondary if there are multiple phases)
• introduces the "particle phases" option

No silicon parameters are added, this PR just tests with a two phase graphite/graphite system

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

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[codecov]

Codecov Report

Merging #2248 (0c4b972) into develop (1472d97) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2248      +/-   ##
===========================================
+ Coverage    99.36%   99.38%   +0.01%     
===========================================
  Files          357      359       +2     
  Lines        19514    19821     +307     
===========================================
+ Hits         19391    19699     +308     
+ Misses         123      122       -1     

Impacted Files	Coverage Δ
...bamm/models/full_battery_models/lithium_ion/mpm.py	100.00% <ø> (ø)
...els/submodels/interface/lithium_plating/plating.py	100.00% <ø> (ø)
pybamm/expression_tree/averages.py	100.00% <100.00%> (ø)
pybamm/expression_tree/broadcasts.py	100.00% <100.00%> (ø)
pybamm/expression_tree/independent_variable.py	100.00% <100.00%> (ø)
pybamm/expression_tree/parameter.py	100.00% <100.00%> (ø)
pybamm/expression_tree/printing/print_name.py	100.00% <100.00%> (ø)
pybamm/geometry/battery_geometry.py	100.00% <100.00%> (ø)
pybamm/geometry/standard_spatial_vars.py	100.00% <100.00%> (ø)
...m/models/full_battery_models/base_battery_model.py	99.78% <100.00%> (+0.02%)	⬆️
... and 67 more

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[brosaplanella]

Looks great, thanks @tinosulzer! Just a few minor questions and comments