Issue 1748 stress induced diffusion

[rtimms]

Description

Add a separate "stress-induced diffusion" model option. Previously this effect was hard-coded into the parameters when the user selected any of the "particle mechanics" options other than "none".

Fixes #1748

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

[valentinsulzer]

Thanks, looks good!

[codecov]

Codecov Report

Merging #1797 (58922b5) into develop (6e887ae) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1797   +/-   ##
========================================
  Coverage    99.24%   99.24%           
========================================
  Files          343      344    +1     
  Lines        18983    19037   +54     
========================================
+ Hits         18840    18894   +54     
  Misses         143      143           

Impacted Files	Coverage Δ
...s/full_battery_models/lithium_ion/newman_tobias.py	100.00% <ø> (ø)
...bamm/models/full_battery_models/lithium_ion/spm.py	100.00% <ø> (ø)
pybamm/models/submodels/base_submodel.py	100.00% <ø> (ø)
...m/models/full_battery_models/base_battery_model.py	99.71% <100.00%> (+<0.01%)	⬆️
...bamm/models/full_battery_models/lithium_ion/dfn.py	100.00% <100.00%> (ø)
pybamm/models/standard_variables.py	100.00% <100.00%> (ø)
pybamm/models/submodels/particle/base_particle.py	100.00% <100.00%> (ø)
...els/submodels/particle/no_distribution/__init__.py	100.00% <100.00%> (ø)
...submodels/particle/no_distribution/base_fickian.py	100.00% <100.00%> (ø)
...dels/particle/no_distribution/fickian_diffusion.py	100.00% <100.00%> (ø)
... and 6 more

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