Issue 1429 simplifications

[valentinsulzer]

Description

Some more automatic simplifications of the expression tree
Should not be part of v0.4.0, do not merge before #1448

Fixes #1429

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 #1449 (d6d36c0) into develop (79236f1) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1449      +/-   ##
===========================================
+ Coverage    98.30%   98.34%   +0.04%     
===========================================
  Files          281      281              
  Lines        16313    16469     +156     
===========================================
+ Hits         16036    16197     +161     
+ Misses         277      272       -5     

Impacted Files	Coverage Δ
pybamm/discretisations/discretisation.py	99.78% <100.00%> (ø)
pybamm/expression_tree/array.py	100.00% <100.00%> (ø)
pybamm/expression_tree/binary_operators.py	99.66% <100.00%> (+0.04%)	⬆️
pybamm/expression_tree/broadcasts.py	100.00% <100.00%> (ø)
pybamm/expression_tree/concatenations.py	97.79% <100.00%> (+0.17%)	⬆️
pybamm/expression_tree/functions.py	100.00% <100.00%> (ø)
pybamm/expression_tree/matrix.py	100.00% <100.00%> (ø)
pybamm/expression_tree/symbol.py	98.17% <100.00%> (+0.05%)	⬆️
pybamm/expression_tree/unary_operators.py	100.00% <100.00%> (ø)
...models/full_battery_models/lead_acid/basic_full.py	100.00% <100.00%> (ø)
... and 32 more

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[brosaplanella]

Amazing work, thanks Tino!