Issue 804 initialize casadi

[valentinsulzer]

Description

Add the (now default) option to use casadi's rootfinder to initialize the DAEs

Fixes #804

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 #844 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #844      +/-   ##
==========================================
+ Coverage   98.24%   98.25%   +0.01%     
==========================================
  Files         180      180              
  Lines       10254    10316      +62     
==========================================
+ Hits        10074    10136      +62     
  Misses        180      180

Impacted Files	Coverage Δ
pybamm/solvers/idaklu_solver.py	89.21% <ø> (ø)	⬆️
pybamm/solvers/scikits_dae_solver.py	98.07% <ø> (ø)	⬆️
pybamm/solvers/casadi_solver.py	100% <ø> (ø)	⬆️
pybamm/solvers/base_solver.py	99.42% <100%> (+0.03%)	⬆️
...odels/full_battery_models/lithium_ion/basic_dfn.py	100% <0%> (ø)	⬆️
...odels/full_battery_models/lithium_ion/basic_spm.py	100% <0%> (ø)	⬆️

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[rtimms]

looks great, thanks @tinosulzer