#977 make casadi solver default

[rtimms]

Description

Makes CasadiSolver the default for DAE models

Fixes #977

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 #978 into develop will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #978      +/-   ##
===========================================
- Coverage    97.48%   97.47%   -0.01%     
===========================================
  Files          222      222              
  Lines        11791    11785       -6     
===========================================
- Hits         11494    11488       -6     
  Misses         297      297              

Impacted Files	Coverage Δ
...l_battery_models/lead_acid/base_lead_acid_model.py	100.00% <ø> (ø)
pybamm/models/base_model.py	98.78% <100.00%> (-0.01%)	⬇️
...ollector/effective_resistance_current_collector.py	96.29% <100.00%> (ø)

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