Issue 966 exchange current

[valentinsulzer]

Description

Make "exchange-current density" a parameter instead of "reaction rate".
Also replaced "surface area density" with "surface area to volume ratio"

Fixes #966

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 #975 into develop will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #975      +/-   ##
===========================================
+ Coverage    97.48%   97.50%   +0.02%     
===========================================
  Files          222      225       +3     
  Lines        11791    11830      +39     
===========================================
+ Hits         11494    11535      +41     
+ Misses         297      295       -2     

Impacted Files	Coverage Δ
...dels/submodels/interface/kinetics/base_kinetics.py	100.00% <ø> (ø)
pybamm/solvers/casadi_solver.py	100.00% <ø> (ø)
...er2019/lead_exchange_current_density_Sulzer2019.py	100.00% <100.00%> (ø)
...ead_dioxide_exchange_current_density_Sulzer2019.py	100.00% <100.00%> (ø)
...2019/oxygen_exchange_current_density_Sulzer2019.py	100.00% <100.00%> (ø)
...0_electrolyte_exchange_current_density_Chen2020.py	100.00% <100.00%> (ø)
..._electrolyte_exchange_current_density_Ecker2015.py	100.00% <100.00%> (ø)
...te_electrolyte_exchange_current_density_Kim2011.py	100.00% <100.00%> (ø)
..._electrolyte_exchange_current_density_PeymanMPM.py	100.00% <100.00%> (ø)
...ectrolyte_exchange_current_density_Dualfoil1998.py	100.00% <100.00%> (ø)
... and 24 more

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[brosaplanella]

Looks great, thanks Tino! Just a minor comment about some commented lines (do we want to leave them there?).

[brosaplanella]

Why are these lines left commented?

[valentinsulzer]

No good reason, I will delete them