Issue 609 broadcast to cell edges

[valentinsulzer]

Description

Add BroadcastToEdge object and edit fluxes in relevant submodels to use this.
Also edited the particle submodels, hopefully a bit clearer now.

Fixes #609

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 #891 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #891      +/-   ##
===========================================
+ Coverage    97.79%   97.79%   +<.01%     
===========================================
  Files          210      207       -3     
  Lines        10603    10631      +28     
===========================================
+ Hits         10369    10397      +28     
  Misses         234      234

Impacted Files	Coverage Δ
...well/diffusion/leading_stefan_maxwell_diffusion.py	100% <100%> (ø)	⬆️
...models/submodels/thermal/x_lumped/base_x_lumped.py	100% <100%> (ø)	⬆️
...bamm/models/full_battery_models/lithium_ion/dfn.py	100% <100%> (ø)	⬆️
pybamm/expression_tree/independent_variable.py	100% <100%> (ø)	⬆️
...m/models/submodels/particle/fast_many_particles.py	100% <100%> (ø)
.../models/submodels/particle/fast_single_particle.py	100% <100%> (ø)
pybamm/expression_tree/broadcasts.py	100% <100%> (ø)	⬆️
pybamm/spatial_methods/spatial_method.py	95.95% <100%> (+0.17%)	⬆️
...amm/models/submodels/oxygen_diffusion/no_oxygen.py	100% <100%> (ø)	⬆️
pybamm/geometry/standard_spatial_vars.py	100% <100%> (ø)	⬆️
... and 12 more

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[rtimms]

looks good thanks!

[rtimms]

why can't we average on edges? is it just that it isn't implemented? (if so, no need to do this now. it can be a separate issue if/when we need it)

[valentinsulzer]

No reason why we can't in theory. I'm not sure what the proper implementation would be in a finite volume context though? For nodes it's clearly just sum all the values, but for edges I don't know what it should be: maybe N[-1] - N[0]?