Issue 762 casadi mass

[rtimms]

Description

Automatically computes the inverse of the mass matrix during discretisation so that the resulting system of DAEs can be provided in semi explicit form, which is required by the casadi solver

Fixes #762

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 #769 into master will increase coverage by <.01%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##           master     #769      +/-   ##
==========================================
+ Coverage   98.38%   98.38%   +<.01%     
==========================================
  Files         179      179              
  Lines        9894     9916      +22     
==========================================
+ Hits         9734     9756      +22     
  Misses        160      160

Impacted Files	Coverage Δ
pybamm/models/base_model.py	98.65% <100%> (+0.03%)	⬆️
pybamm/discretisations/discretisation.py	99.74% <100%> (ø)	⬆️
pybamm/solvers/dae_solver.py	100% <100%> (ø)	⬆️
pybamm/solvers/ode_solver.py	98.08% <100%> (+0.01%)	⬆️
pybamm/solvers/casadi_solver.py	97.67% <75%> (ø)	⬆️

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[valentinsulzer]

Looks good, thanks @rtimms . Don't forget to update changelog