Issue 1816 domain error

[valentinsulzer]

Description

Some changes to the expression tree simplification logic.
Fixes #1816

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

[brosaplanella]

Looks good, happy to merge when tests pass. Let's remember to add the CHANGELOG and add dion for bug reports.

[codecov]

Codecov Report

Merging #1831 (8de3ee9) into develop (18449d7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1831   +/-   ##
========================================
  Coverage    99.26%   99.26%           
========================================
  Files          345      345           
  Lines        18900    18944   +44     
========================================
+ Hits         18761    18805   +44     
  Misses         139      139           

Impacted Files	Coverage Δ
pybamm/discretisations/discretisation.py	99.78% <ø> (ø)
pybamm/expression_tree/binary_operators.py	99.55% <100.00%> (+0.03%)	⬆️
pybamm/expression_tree/vector.py	100.00% <100.00%> (ø)
pybamm/spatial_methods/finite_volume.py	98.12% <100.00%> (-0.03%)	⬇️

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