Issue 744 particledistribution

[suhaklee]

Description

Added particle size distribution in x to the DFN model.

Fixes #744

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

Merging #745 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #745      +/-   ##
==========================================
+ Coverage   98.38%   98.39%   +<.01%     
==========================================
  Files         176      176              
  Lines        9678     9701      +23     
==========================================
+ Hits         9522     9545      +23     
  Misses        156      156

Impacted Files	Coverage Δ
...ubmodels/particle/fickian/base_fickian_particle.py	100% <ø> (ø)	⬆️
...bmodels/particle/fickian/fickian_many_particles.py	100% <100%> (ø)	⬆️
pybamm/parameters/parameter_values.py	100% <100%> (ø)	⬆️
pybamm/expression_tree/broadcasts.py	94.02% <100%> (+0.69%)	⬆️
...models/particle/fickian/fickian_single_particle.py	100% <100%> (ø)	⬆️

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[rtimms]

looks great, thanks!

[rtimms]

is it worth having a comment here just to make it clear that this is a size distribution in x, rather than a distribution of particle sizes at each x?