#961 fix interpolant ids

[valentinsulzer]

Description

Change interpolant id so that different interpolants have different ids

Fixes #961

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 #962 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #962   +/-   ##
========================================
  Coverage    98.11%   98.11%           
========================================
  Files          221      221           
  Lines        11518    11528   +10     
========================================
+ Hits         11301    11311   +10     
  Misses         217      217           

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

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