speed up electrode SOH calculations

[valentinsulzer]

Description

Speed up some performance regressions introduced in #2192

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 #2199 (4d40bb6) into develop (4c42312) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #2199   +/-   ##
========================================
  Coverage    99.36%   99.36%           
========================================
  Files          355      355           
  Lines        19451    19506   +55     
========================================
+ Hits         19327    19382   +55     
  Misses         124      124           

Impacted Files	Coverage Δ
...models/full_battery_models/lithium_ion/__init__.py	100.00% <ø> (ø)
...s/full_battery_models/lithium_ion/electrode_soh.py	100.00% <100.00%> (ø)
pybamm/simulation.py	98.83% <100.00%> (+0.01%)	⬆️
pybamm/solvers/solution.py	100.00% <100.00%> (ø)

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[valentinsulzer]

Would be good to merge this before the new release

[brosaplanella]

Looks good, feel free to merge once you have updated the CHANGELOG

[brosaplanella]

Suggested change

	# # Voltage variables
	# Voltage variables