Issue 963 misc bugs

[valentinsulzer]

Description

• Add some extra checks when creating model
• Make grad (broadcast) and div(broadcast) automatically return broadcast(0)
• don't allow rediscretising a model once discretised (if allowed, it usually fails anyway)

Fixes #963

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 #971 into develop will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #971      +/-   ##
===========================================
+ Coverage    97.36%   97.52%   +0.16%     
===========================================
  Files          222      222              
  Lines        11689    12001     +312     
===========================================
+ Hits         11381    11704     +323     
+ Misses         308      297      -11     

Impacted Files	Coverage Δ
...amm/models/submodels/convection/base_convection.py	100.00% <ø> (+27.50%)	⬆️
pybamm/simulation.py	98.41% <ø> (ø)
pybamm/discretisations/discretisation.py	99.77% <100.00%> (+<0.01%)	⬆️
pybamm/expression_tree/functions.py	100.00% <100.00%> (ø)
pybamm/expression_tree/operations/simplify.py	95.14% <100.00%> (+0.05%)	⬆️
pybamm/expression_tree/symbol.py	97.46% <100.00%> (+0.01%)	⬆️
pybamm/expression_tree/unary_operators.py	96.51% <100.00%> (+0.12%)	⬆️
pybamm/meshes/meshes.py	98.93% <100.00%> (+0.02%)	⬆️
pybamm/models/base_model.py	98.78% <100.00%> (+0.11%)	⬆️
pybamm/models/submodels/base_submodel.py	91.93% <100.00%> (-0.72%)	⬇️
... and 11 more

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[rtimms]

great, thanks!

[rtimms]

looks like this is out of date with the changes to casadi, but should be fixed by merging

[valentinsulzer]

Ah thanks, I was wondering why the tests were failing