Re-solve before root finding when handling events in casadi

[rtimms]

Description

Currently when an event is triggered in casadi we construct a linear interpolant, use a root finding algorithm to find the event time, and then go back and solve again up to the event time. This can give the wrong event time when the linear interpolant isn't a good fit to the actual solution (see #1318).

This PR changes it so that when an event is triggered we first go back and re-solve in the current time window using a more dense array of times. This solution is then used to construct a higher-order interpolant for use in the root finding algorithm. This allows the event time to be found more accurately.

I'm leaving #1318 open as there are some other (more sophisticated) methods for handling events that could be explored.

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 #1328 (1c40966) into develop (948505e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1328   +/-   ##
========================================
  Coverage    98.12%   98.12%           
========================================
  Files          272      272           
  Lines        15439    15443    +4     
========================================
+ Hits         15150    15154    +4     
  Misses         289      289           

Impacted Files	Coverage Δ
pybamm/solvers/base_solver.py	99.14% <100.00%> (ø)
pybamm/solvers/casadi_solver.py	99.54% <100.00%> (+<0.01%)	⬆️

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[valentinsulzer]

thanks @rtimms !