Issue 1575 discharge energy

[valentinsulzer]

Description

Add "Discharge energy [W.h]" as an output variable (integral of power).
Set option "calculate discharge energy" to "true" to turn on. It's false by default as calculating it can be slow.

Fixes #1575

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 #1969 (7d28451) into develop (9a1f455) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1969   +/-   ##
========================================
  Coverage    99.32%   99.32%           
========================================
  Files          346      346           
  Lines        19064    19078   +14     
========================================
+ Hits         18936    18950   +14     
  Misses         128      128           

Impacted Files	Coverage Δ
...ybamm/models/full_battery_models/lead_acid/loqs.py	100.00% <ø> (ø)
pybamm/simulation.py	98.36% <ø> (ø)
...m/models/full_battery_models/base_battery_model.py	99.76% <100.00%> (ø)
pybamm/models/standard_variables.py	100.00% <100.00%> (ø)
...ubmodels/external_circuit/base_external_circuit.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%> (ø)
pybamm/models/submodels/thermal/isothermal.py	100.00% <100.00%> (ø)

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[TomTranter]

@tinosulzer this is a lot of code changes just for one calculation. Did it really slow things down so much to warrant making it an option?

[valentinsulzer]

Here are the benchmarks with the discharge energy calculation: https://github.com/pybamm-team/PyBaMM/runs/5458023045?check_suite_focus=true

It could probably be done as a post-processing step, with low overhead, but that would be a bigger change

[valentinsulzer]

It's not that many changes? Most of them are just passing options to the external circuit submodels, which we might want to do at some point later anyway

[TomTranter]

ok if you think might be useful anyway. I just prefer not having too many options