Issue 830 algebraic

[valentinsulzer]

Description

Reformat algebraic solver to be more in line with the other solvers, and add casadi algebraic solver.
We could call the algebraic solvers to initialize the dae models, would reduce code duplication, but might be a little bit confusing?

Fixes #830

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 #868 into develop will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #868      +/-   ##
===========================================
+ Coverage    97.70%   97.75%   +0.04%     
===========================================
  Files          201      202       +1     
  Lines        10433    10459      +26     
===========================================
+ Hits         10194    10224      +30     
+ Misses         239      235       -4     

Impacted Files	Coverage Δ
pybamm/solvers/algebraic_solver.py	100.00% <100.00%> (+5.12%)	⬆️
pybamm/solvers/base_solver.py	99.72% <100.00%> (+0.29%)	⬆️
pybamm/solvers/casadi_algebraic_solver.py	100.00% <100.00%> (ø)
pybamm/solvers/casadi_solver.py	100.00% <100.00%> (ø)
pybamm/models/base_model.py	97.81% <0.00%> (-0.44%)	⬇️
pybamm/expression_tree/binary_operators.py	93.10% <0.00%> (-0.32%)	⬇️
pybamm/solvers/idaklu_solver.py	89.42% <0.00%> (+0.96%)	⬆️

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[rtimms]

thanks!

[rtimms]

should we need to raise an error here if t_eval is None and the solver isn't algebraic?