Add three dimensional interpolation

[Scottmar93]

Description

Extended the interpolation class to allow 3D interpolants utiliizing scipys RegularGridInterpolator and allowed for conversion to casadi. I have added an minimal example script where the 3D interpolation is used in a model and solved. If you are happy with the coverage of the tests already then we can remove it or move to the integration tests.

Fixes #

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

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Codecov Report

Base: 99.66% // Head: 99.66% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (c9e2aca) compared to base (58879c8).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2380   +/-   ##
========================================
  Coverage    99.66%   99.66%           
========================================
  Files          269      269           
  Lines        20145    20190   +45     
========================================
+ Hits         20078    20123   +45     
  Misses          67       67           

Impacted Files	Coverage Δ
pybamm/expression_tree/interpolant.py	100.00% <100.00%> (ø)
...mm/expression_tree/operations/convert_to_casadi.py	100.00% <100.00%> (ø)

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[valentinsulzer]

Looks good, just needs coverage

[Scottmar93]

Currently updating to cover cases where nans and inconsistently shaped children are passed to the RGI (unfortunately it cannot handle these cases unlike interp2d and interp1d)

[valentinsulzer]

Can you merge develop to see if that fixes benchmarks?

[Scottmar93]

Seems to fix it thanks!