Issue 1221 convert to casadi

[valentinsulzer]

Description

Convert variables to casadi for faster post-processing.
The conversion is done the first time the variable is called, and the casadi function is then stored in the model (in model._variables_casadi. Therefore, when doing repeated solutions of the same model (e.g. for parameter sweeps) the conversion is only performed the first time.

Fixes #1221

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 #1316 (4b84f39) into develop (66b9589) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1316   +/-   ##
========================================
  Coverage    98.10%   98.10%           
========================================
  Files          272      272           
  Lines        15277    15290   +13     
========================================
+ Hits         14988    15001   +13     
  Misses         289      289           

Impacted Files	Coverage Δ
pybamm/models/base_model.py	99.11% <100.00%> (+0.03%)	⬆️
pybamm/simulation.py	96.91% <100.00%> (-0.05%)	⬇️
pybamm/solvers/base_solver.py	99.04% <100.00%> (+<0.01%)	⬆️
pybamm/solvers/casadi_solver.py	99.47% <100.00%> (-0.01%)	⬇️
pybamm/solvers/processed_symbolic_variable.py	100.00% <100.00%> (ø)
pybamm/solvers/processed_variable.py	99.59% <100.00%> (-0.03%)	⬇️
pybamm/solvers/solution.py	99.43% <100.00%> (+0.05%)	⬆️
pybamm/expression_tree/unary_operators.py	99.79% <0.00%> (-0.21%)	⬇️
pybamm/expression_tree/operations/evaluate.py	98.09% <0.00%> (ø)
pybamm/solvers/idaklu_solver.py	90.26% <0.00%> (+0.88%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 66b9589...4b84f39. Read the comment docs.

[brosaplanella]

Looks good to me, happy to merge once the test passes. Thanks Tino!

[brosaplanella]

This test fails but I can't understand why it doesn't take the .full().

[valentinsulzer]

get_termination_reason had changed in #1300 and was returning a numpy array