Issue 684 casadi events

[valentinsulzer]

Description

Add 'safe' mode to casadi for solving with events. This is hacky and make the solver slower.

Fixes #684

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.

• 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

[Scottmar93]

Looks great @tinosulzer. Well done getting this up and running. Looks like still getting some NaNs in the solution though. Maybe we can just remove those? A bit hacky though

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@44614d1). Click here to learn what that means.
The diff coverage is 90.54%.

@@            Coverage Diff            @@
##             master     #691   +/-   ##
=========================================
  Coverage          ?   98.21%           
=========================================
  Files             ?      176           
  Lines             ?     9040           
  Branches          ?        0           
=========================================
  Hits              ?     8879           
  Misses            ?      161           
  Partials          ?        0

Impacted Files	Coverage Δ
pybamm/solvers/idaklu_solver.py	88.4% <100%> (ø)
pybamm/solvers/dae_solver.py	99.45% <100%> (ø)
pybamm/solvers/ode_solver.py	100% <100%> (ø)
pybamm/solvers/base_solver.py	97.4% <100%> (ø)
pybamm/solvers/scikits_ode_solver.py	98.24% <100%> (ø)
pybamm/solvers/scikits_dae_solver.py	97.72% <100%> (ø)
pybamm/solvers/algebraic_solver.py	100% <100%> (ø)
pybamm/quick_plot.py	96.75% <100%> (ø)
pybamm/solvers/solution.py	100% <100%> (ø)
pybamm/solvers/scipy_solver.py	100% <100%> (ø)
... and 1 more

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