#1251 fix function parameter diff bug

[valentinsulzer]

Description

Fixes #1251

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 #1294 (381351a) into develop (ee3d67c) will increase coverage by 0.00%.
The diff coverage is 97.72%.

@@           Coverage Diff            @@
##           develop    #1294   +/-   ##
========================================
  Coverage    98.15%   98.15%           
========================================
  Files          272      272           
  Lines        15600    15622   +22     
========================================
+ Hits         15312    15334   +22     
  Misses         288      288           

Impacted Files	Coverage Δ
pybamm/expression_tree/variable.py	100.00% <ø> (ø)
pybamm/solvers/base_solver.py	99.13% <66.66%> (ø)
pybamm/discretisations/discretisation.py	99.78% <100.00%> (+<0.01%)	⬆️
pybamm/expression_tree/binary_operators.py	97.49% <100.00%> (+0.01%)	⬆️
pybamm/expression_tree/symbol.py	98.57% <100.00%> (ø)
pybamm/expression_tree/unary_operators.py	99.79% <100.00%> (+<0.01%)	⬆️
...dels/submodels/interface/kinetics/base_kinetics.py	100.00% <100.00%> (ø)
pybamm/parameters/parameter_values.py	99.38% <100.00%> (+<0.01%)	⬆️
pybamm/solvers/casadi_algebraic_solver.py	98.85% <100.00%> (+0.13%)	⬆️
pybamm/solvers/casadi_solver.py	99.52% <100.00%> (-0.02%)	⬇️

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[valentinsulzer]

Coverage error is something that was already failing before ...

[brosaplanella]

Looks good to me, thanks Tino!

[brosaplanella]

Can this line be removed now?