Issue 1310 interpolant

[valentinsulzer]

Description

Add some funcitonality to Interpolant to take care of more cases.

Fixes #1310

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 #1312 (5e6c3d5) into develop (c573c78) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1312   +/-   ##
========================================
  Coverage    98.10%   98.10%           
========================================
  Files          272      272           
  Lines        15229    15270   +41     
========================================
+ Hits         14940    14981   +41     
  Misses         289      289           

Impacted Files	Coverage Δ
...ollector/effective_resistance_current_collector.py	96.42% <ø> (ø)
pybamm/discretisations/discretisation.py	99.78% <100.00%> (ø)
pybamm/expression_tree/interpolant.py	100.00% <100.00%> (ø)
...mm/expression_tree/operations/convert_to_casadi.py	100.00% <100.00%> (ø)
...i2020/graphite_entropy_Enertech_Ai2020_function.py	100.00% <100.00%> (ø)
...L_Mohtat2020/graphite_entropic_change_PeymanMPM.py	100.00% <100.00%> (ø)
..._Marquis2019/graphite_entropic_change_Moura2016.py	100.00% <100.00%> (ø)
...C_UMBL_Mohtat2020/NMC_entropic_change_PeymanMPM.py	100.00% <100.00%> (ø)
...o2_Ai2020/lico2_entropic_change_Ai2020_function.py	100.00% <100.00%> (ø)
...co2_Marquis2019/lico2_entropic_change_Moura2016.py	100.00% <100.00%> (ø)
... and 3 more

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[brosaplanella]

Looks great, thanks Tino!