Issue 600 interpolate

[valentinsulzer]

Description

Add Interpolant class to interpolate experimental data (e.g. OCP curves).

Fixes #600

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

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
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

Merging #661 into issue-563-parameters-by-material will increase coverage by <.01%.
The diff coverage is 100%.

@@                         Coverage Diff                          @@
##           issue-563-parameters-by-material     #661      +/-   ##
====================================================================
+ Coverage                             98.52%   98.53%   +<.01%     
====================================================================
  Files                                   168      169       +1     
  Lines                                  8226     8255      +29     
====================================================================
+ Hits                                   8105     8134      +29     
  Misses                                  121      121

Impacted Files	Coverage Δ
pybamm/parameters/parameter_values.py	99.47% <100%> (+0.01%)	⬆️
pybamm/expression_tree/functions.py	100% <100%> (ø)	⬆️
pybamm/expression_tree/simplify.py	94.98% <100%> (ø)	⬆️
pybamm/expression_tree/interpolant.py	100% <100%> (ø)

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[brosaplanella]

Maybe we should agree on a format for the data (like headers, where the data comes from...).

[brosaplanella]

Looks good to me. Thanks @tinosulzer!