Issue 846 bugs

[valentinsulzer]

Description

Fix miscellaneous bugs for implementing fuel cell models

Fixes #846
Fixes #742
Fixes #735

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 #876 into develop will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #876      +/-   ##
===========================================
+ Coverage    97.76%   97.79%   +0.03%     
===========================================
  Files          210      210              
  Lines        10523    10585      +62     
===========================================
+ Hits         10288    10352      +64     
+ Misses         235      233       -2

Impacted Files	Coverage Δ
pybamm/expression_tree/independent_variable.py	100% <ø> (ø)	⬆️
pybamm/expression_tree/functions.py	100% <ø> (ø)	⬆️
pybamm/expression_tree/exceptions.py	100% <ø> (ø)	⬆️
pybamm/expression_tree/symbol.py	96.99% <ø> (ø)	⬆️
pybamm/parameters/parameter_values.py	100% <100%> (ø)	⬆️
pybamm/solvers/casadi_algebraic_solver.py	100% <100%> (ø)	⬆️
pybamm/quick_plot.py	98.86% <100%> (ø)	⬆️
pybamm/expression_tree/unary_operators.py	96.56% <100%> (+0.41%)	⬆️
...mm/expression_tree/operations/convert_to_casadi.py	92.85% <100%> (+0.43%)	⬆️
pybamm/expression_tree/operations/evaluate.py	95.83% <100%> (+0.18%)	⬆️
... and 4 more

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[rtimms]

looks good thanks!