reformat BaseSolver.set_up with shorter functions

[valentinsulzer]

Description

Clean up the BaseSolver.set_up function to make it a bit easier to read

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 #2209 (b6dded8) into develop (9126986) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2209      +/-   ##
===========================================
+ Coverage    99.37%   99.39%   +0.02%     
===========================================
  Files          365      365              
  Lines        20046    20077      +31     
===========================================
+ Hits         19920    19955      +35     
+ Misses         126      122       -4     

Impacted Files	Coverage Δ
pybamm/parameters/parameter_sets.py	100.00% <ø> (ø)
pybamm/solvers/base_solver.py	100.00% <100.00%> (ø)
pybamm/models/submodels/base_submodel.py	100.00% <0.00%> (ø)
...m/models/full_battery_models/base_battery_model.py	99.78% <0.00%> (+0.64%)	⬆️
...models/submodels/interface/kinetics/no_reaction.py	100.00% <0.00%> (+3.84%)	⬆️

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[valentinsulzer]

@martinjrobins could you review this to make sure it's ok? Shouldn't change any functionality

[brosaplanella]

Looks good, thanks Valentin!