Issue 874 simplify thermal

[rtimms]

Description

Simplifies the thermal submodels. Now we only have models for (option string name in brackets):

• Isothermal ("isothermal")
• Lumped ("lumped"),
• OneDimensionalX ("x-full")
• `CurrentCollector1D ("x-lumped" with "dimensionality": 1)
• `CurrentCollector2D ("x-lumped" with "dimensionality": 2)

The option "xyz-lumped" has been removed. Now "lumped" always returns an ODE for the volume-averaged temperature.

The option "thermal current collectors" has also been removed. All models now account for the presence of current collectors.

Fixes #874

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 #938 into develop will increase coverage by 0.01%.
The diff coverage is 99.36%.

@@             Coverage Diff             @@
##           develop     #938      +/-   ##
===========================================
+ Coverage    98.10%   98.12%   +0.01%     
===========================================
  Files          223      214       -9     
  Lines        11258    11123     -135     
===========================================
- Hits         11045    10914     -131     
+ Misses         213      209       -4     

Impacted Files	Coverage Δ
...dels/submodels/current_collector/potential_pair.py	100.00% <ø> (ø)
pybamm/models/submodels/thermal/isothermal.py	91.30% <85.71%> (ø)
...m/models/full_battery_models/base_battery_model.py	100.00% <100.00%> (ø)
pybamm/models/submodels/thermal/__init__.py	100.00% <100.00%> (ø)
pybamm/models/submodels/thermal/base_thermal.py	100.00% <100.00%> (+7.05%)	⬆️
pybamm/models/submodels/thermal/lumped.py	100.00% <100.00%> (ø)
...mm/models/submodels/thermal/pouch_cell/__init__.py	100.00% <100.00%> (ø)
...mal/pouch_cell/pouch_cell_1D_current_collectors.py	100.00% <100.00%> (ø)
...mal/pouch_cell/pouch_cell_2D_current_collectors.py	100.00% <100.00%> (ø)
pybamm/models/submodels/thermal/x_full.py	100.00% <100.00%> (ø)
... and 4 more

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[valentinsulzer]

Looks good to me, much cleaner now! Makes sense for @Scottmar93 to review too as he is more familiar with these models
There is the odd "domian" and "defualt" , but I don't care about that ..

[Scottmar93]

I think this is all much clearer now. Make sense to leave changing how grad works for another issue and it'll be really clean after that!

I think x_full.py and x_lumped.py as still left though, should these be deleted?