Issue 2382 simulation only

[valentinsulzer]

Description

Reformat Simulation and Experiment classes to process models more cleanly and efficiently.
Progress towards #2322 but not yet complete

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

Base: 99.72% // Head: 99.71% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (e1d36fa) compared to base (4127bd1).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2395      +/-   ##
===========================================
- Coverage    99.72%   99.71%   -0.01%     
===========================================
  Files          268      267       -1     
  Lines        20153    20044     -109     
===========================================
- Hits         20097    19987     -110     
- Misses          56       57       +1     

Impacted Files	Coverage Δ
pybamm/experiments/experiment.py	100.00% <100.00%> (ø)
...ernal_circuit/function_control_external_circuit.py	98.92% <100.00%> (+0.01%)	⬆️
...dels/submodels/interface/kinetics/base_kinetics.py	100.00% <100.00%> (ø)
pybamm/simulation.py	100.00% <100.00%> (ø)
pybamm/solvers/base_solver.py	100.00% <100.00%> (ø)
pybamm/expression_tree/parameter.py	98.76% <0.00%> (-1.24%)	⬇️

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[brosaplanella]

Looks good, thanks!