Issue 759 non smooth dae

[martinjrobins]

Description

Fixes #759

• adds pybamm.Event class for holding information about events
• added a new class of event that returns the time that a discontinuity in the solution occurs
• changed the BaseSolver to filter out the discontinuity events. Integration is split over multiple segments of the time series, new time points are added just before and just after the discontinuity and added to the solution object
• for models using heavyside functions in certain patterns, the discretisation class automatically adds in discontinuity events required

Type of change

• New feature (non-breaking change which adds functionality)

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 #808 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #808   +/-   ##
=======================================
  Coverage   98.14%   98.14%           
=======================================
  Files         179      180    +1     
  Lines        9919     9987   +68     
=======================================
+ Hits         9735     9802   +67     
- Misses        184      185    +1     

Impacted Files	Coverage Δ
pybamm/models/event.py	90.47% <0.00%> (ø)
pybamm/expression_tree/functions.py	100.00% <0.00%> (+0.58%)	⬆️

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[valentinsulzer]

Thanks @martinjrobins , looks great as is! Can you add this to changelog? We will need to check that the DFN works with discontinuous current input now, but I think this can be something for a separate issue (and also setting up interpolants with heaviside)