Issue 784 simplify solver

[valentinsulzer]

Description

Simplify the various solver classes by merging the set_up methods and getting rid of compute_solution. Also combined the implementations for external variables and inputs

Fixes #784
Fixes #789
Fixes #463

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 #800 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #800      +/-   ##
==========================================
+ Coverage   98.29%   98.41%   +0.11%     
==========================================
  Files         177      179       +2     
  Lines        9746     9910     +164     
==========================================
+ Hits         9580     9753     +173     
+ Misses        166      157       -9

Impacted Files	Coverage Δ
pybamm/models/submodels/particle/base_particle.py	97.29% <ø> (ø)	⬆️
pybamm/parameters/parameter_values.py	100% <100%> (ø)	⬆️
pybamm/solvers/idaklu_solver.py	88.23% <100%> (+3.92%)	⬆️
pybamm/solvers/casadi_solver.py	100% <100%> (+1.17%)	⬆️
pybamm/util.py	100% <100%> (ø)	⬆️
pybamm/parameters/print_parameters.py	96.96% <100%> (ø)	⬆️
...models/full_battery_models/lithium_ion/__init__.py	100% <100%> (ø)	⬆️
...odels/full_battery_models/lithium_ion/basic_dfn.py	100% <100%> (ø)
.../models/submodels/thermal/isothermal/isothermal.py	100% <100%> (ø)	⬆️
pybamm/expression_tree/variable.py	100% <100%> (+7.89%)	⬆️
... and 10 more

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[Scottmar93]

Looks really good @tinosulzer. Certainly much simpler to work out what is going on in the solvers now by getting rid of the DAESolver and ODESolver classes and those multiple set_ups!! Also really like that you have tidied up external variables and have them and the input parameters treated in a similar way.

[Scottmar93]

Does external variables actually work when just solving? I thought it was only implemented for "stepping"

[valentinsulzer]

Yes, it should work identically with both now - though I guess the main use cases will be for stepping