add ocp as a separate submodel

[valentinsulzer]

Description

Adds OCV as a submodel. Currently there is only one kind of submodel, but #2073 will be adding the current sigmoid.

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

[rtimms]

thanks @tinosulzer looks good!

[rtimms]

think you need to move the defn of set_convection_submodel into this file from SPM, SPMe, DFN

[rtimms]

oh, actually it looks like this has broken the half-cell models?

[codecov]

Codecov Report

Merging #2094 (596e318) into develop (4ec415d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #2094   +/-   ##
========================================
  Coverage    99.38%   99.38%           
========================================
  Files          352      355    +3     
  Lines        19289    19302   +13     
========================================
+ Hits         19170    19183   +13     
  Misses         119      119           

Impacted Files	Coverage Δ
...ybamm/models/submodels/interface/base_interface.py	100.00% <ø> (ø)
pybamm/parameters/parameter_sets.py	100.00% <ø> (ø)
pybamm/__init__.py	100.00% <100.00%> (ø)
...l_battery_models/lead_acid/base_lead_acid_model.py	100.00% <100.00%> (ø)
...ybamm/models/full_battery_models/lead_acid/full.py	100.00% <100.00%> (ø)
...dels/full_battery_models/lead_acid/higher_order.py	98.14% <100.00%> (+0.01%)	⬆️
...ybamm/models/full_battery_models/lead_acid/loqs.py	100.00% <100.00%> (ø)
...ttery_models/lithium_ion/base_lithium_ion_model.py	100.00% <100.00%> (ø)
...bamm/models/full_battery_models/lithium_ion/dfn.py	100.00% <100.00%> (ø)
...bamm/models/full_battery_models/lithium_ion/spm.py	100.00% <100.00%> (ø)
... and 9 more

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