Issue 1903 power

[valentinsulzer]

Description

Adds more operating modes for power and resistance control.

The explicit models can only be used with DFN without surface form, so it's not possible to completely eliminate algebraic equations using that (but it can be done using the differential models)

Fixes #1903
Fixes #1691

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 #1917 (cde0406) into develop (4c69ca0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1917   +/-   ##
========================================
  Coverage    99.32%   99.32%           
========================================
  Files          346      346           
  Lines        19002    19058   +56     
========================================
+ Hits         18874    18930   +56     
  Misses         128      128           

Impacted Files	Coverage Δ
...ybamm/models/full_battery_models/lead_acid/loqs.py	100.00% <ø> (ø)
...m/models/full_battery_models/base_battery_model.py	99.76% <100.00%> (+<0.01%)	⬆️
...current_collector/homogeneous_current_collector.py	100.00% <100.00%> (ø)
.../submodels/electrolyte_diffusion/full_diffusion.py	100.00% <100.00%> (ø)
...bamm/models/submodels/external_circuit/__init__.py	100.00% <100.00%> (ø)
...ernal_circuit/explicit_control_external_circuit.py	100.00% <100.00%> (ø)
...ernal_circuit/function_control_external_circuit.py	98.91% <100.00%> (+0.24%)	⬆️
pybamm/simulation.py	98.36% <100.00%> (ø)

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[rtimms]

thanks @tinosulzer looks good!

[rtimms]

need to change the example script back

[rtimms]

the CCCV option isn't listed in the docs

[rtimms]

voltage divided by resistance