#1507 remove algebraic equation check

[rtimms]

Description

Remove the check "each variable in the algebraic eqn keys must appear in the eqn"

Fixes #1507

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 #1510 (e133009) into develop (f6cb07a) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1510      +/-   ##
===========================================
+ Coverage    98.34%   98.40%   +0.05%     
===========================================
  Files          296      296              
  Lines        17149    17787     +638     
===========================================
+ Hits         16866    17504     +638     
  Misses         283      283              

Impacted Files	Coverage Δ
...ollector/effective_resistance_current_collector.py	96.66% <ø> (ø)
...rrent_collector/quite_conductive_potential_pair.py	100.00% <ø> (ø)
pybamm/models/base_model.py	99.22% <100.00%> (-0.02%)	⬇️
pybamm/expression_tree/unary_operators.py	99.79% <0.00%> (-0.21%)	⬇️
pybamm/expression_tree/array.py	100.00% <0.00%> (ø)
pybamm/expression_tree/scalar.py	100.00% <0.00%> (ø)
pybamm/parameters/base_parameters.py	100.00% <0.00%> (ø)
pybamm/expression_tree/binary_operators.py	99.78% <0.00%> (+0.12%)	⬆️
pybamm/solvers/base_solver.py	99.59% <0.00%> (+0.41%)	⬆️

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[valentinsulzer]

Cheers @rtimms . If you're feeling brave, there might be some hacks in the potential pair models that were done to pass this test and can now be reversed. No worries if not.