Allow 2D cubic interpolation

[TomTranter]

Description

Allow cubic interpolation of 2D data

Fixes # 2017

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.

• [x] 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[bot]]

Codecov Report

Merging #2018 (4b21fbe) into develop (dd0d4b5) will decrease coverage by 0.01%. The diff coverage is 90.90%.

@@             Coverage Diff             @@
##           develop    #2018      +/-   ##
===========================================
- Coverage    99.35%   99.34%   -0.02%     
===========================================
  Files          346      346              
  Lines        18972    18978       +6     
===========================================
+ Hits         18850    18854       +4     
- Misses         122      124       +2     

Impacted Files	Coverage Δ
pybamm/expression_tree/interpolant.py	98.85% <90.00%> (-1.15%)	:arrow_down:
...mm/expression_tree/operations/convert_to_casadi.py	100.00% <100.00%> (ø)
pybamm/expression_tree/symbol.py	98.54% <0.00%> (-0.25%)	:arrow_down:

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[rtimms]

Now in #2258