explicitly create interpolants from functions

[valentinsulzer]

Description

In the standard parameter sets, create the interpolants from functions explicitly (instead of relying on the ParameterValues class to do it). This allows finer control of hyperparameters (e.g. interpolator).

Using "cubic" interpolator for OCV gives much faster performance than linear interpolator (e.g. 1s -> 0.5s solve time for Ai2020 on my laptop)

Fixes #2410

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

[Scottmar93]

Looks great cheers

[codecov]

Codecov Report

Base: 99.72% // Head: 99.72% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (a696074) compared to base (0229260).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2510   +/-   ##
========================================
  Coverage    99.72%   99.72%           
========================================
  Files          268      268           
  Lines        19285    19311   +26     
========================================
+ Hits         19232    19258   +26     
  Misses          53       53           

Impacted Files	Coverage Δ
pybamm/parameters/parameter_values.py	100.00% <ø> (ø)
pybamm/input/parameters/ecm/example_set.py	100.00% <100.00%> (ø)
pybamm/input/parameters/lithium_ion/Ai2020.py	100.00% <100.00%> (ø)
...input/parameters/lithium_ion/Chen2020_composite.py	100.00% <100.00%> (ø)
pybamm/input/parameters/lithium_ion/Ecker2015.py	100.00% <100.00%> (ø)
pybamm/input/parameters/lithium_ion/NCA_Kim2011.py	100.00% <100.00%> (ø)
pybamm/input/parameters/lithium_ion/OKane2022.py	100.00% <100.00%> (ø)
...ery_models/equivalent_circuit/ecm_model_options.py	100.00% <100.00%> (ø)
...s/full_battery_models/lithium_ion/electrode_soh.py	100.00% <100.00%> (ø)

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[valentinsulzer]

ready for review (changed a bit since @Scottmar93 reviewed)

[brosaplanella]

Looks great, thanks Valentin! Just a couple of very minor comments.