Issue 623 soc enhancement

[TomTranter]

Description

Calculate SOC.
Introduce r_average operator

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

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
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

Merging #627 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
- Coverage   98.48%   98.48%   -0.01%     
==========================================
  Files         169      167       -2     
  Lines        8204     8172      -32     
==========================================
- Hits         8080     8048      -32     
  Misses        124      124

Impacted Files	Coverage Δ
...bmodels/particle/fickian/fickian_many_particles.py	100% <ø> (ø)	⬆️
pybamm/solvers/base_solver.py	98.48% <ø> (+1.71%)	⬆️
pybamm/expression_tree/unary_operators.py	96.18% <100%> (+0.1%)	⬆️
pybamm/models/submodels/particle/base_particle.py	97.29% <100%> (+0.42%)	⬆️
pybamm/expression_tree/evaluate.py	95.74% <0%> (-1.07%)	⬇️
pybamm/discretisations/discretisation.py	100% <0%> (ø)	⬆️
...m/models/full_battery_models/base_battery_model.py	99.57% <0%> (ø)	⬆️
pybamm/expression_tree/symbol.py	97.58% <0%> (ø)	⬆️
pybamm/models/reaction_diffusion.py
pybamm/models/simple_ode_model.py
... and 2 more

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[rtimms]

looks great to me, thanks @TomTranter ! just a couple of minor comments

[valentinsulzer]

Looks good to me too :)

[TomTranter]

@rtimms @tinosulzer I made the example script a bit longer. You may want to put this in results instead? It's good to go otherwise