Add simulation save and load

[valentinsulzer]

Description

Sort out all pickle issues and add very basic save / load functionality.

Generated python code cannot be pickled so raises error (@martinjrobins any ideas for a way round this?)

Note this branch currently has changes from 731 but 731 is ready be merged to master

Fixes #730

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.

• 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 #732 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #732      +/-   ##
==========================================
+ Coverage   98.18%   98.27%   +0.09%     
==========================================
  Files         175      175              
  Lines        9288     9350      +62     
==========================================
+ Hits         9119     9189      +70     
+ Misses        169      161       -8

Impacted Files	Coverage Δ
pybamm/expression_tree/binary_operators.py	98.84% <100%> (ø)	⬆️
pybamm/solvers/dae_solver.py	99.52% <100%> (+0.07%)	⬆️
pybamm/solvers/ode_solver.py	97.27% <100%> (+0.27%)	⬆️
pybamm/solvers/casadi_solver.py	98.79% <100%> (+0.09%)	⬆️
pybamm/simulation.py	85.61% <100%> (+1.05%)	⬆️
pybamm/parameters/parameter_values.py	100% <100%> (ø)	⬆️
pybamm/discretisations/discretisation.py	99.71% <100%> (-0.01%)	⬇️
pybamm/models/base_model.py	98.51% <100%> (+0.66%)	⬆️
pybamm/expression_tree/operations/evaluate.py	94.68% <0%> (+3.19%)	⬆️
pybamm/solvers/idaklu_solver.py	87.25% <0%> (+3.92%)	⬆️

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[martinjrobins]

at some point the generated python code is just a plain string, I would pickle that instead. You would have to ensure that when you load the pickled object that the string is then converted to python bytecode again

[rtimms]

looks great thanks @tinosulzer

[valentinsulzer]

at some point the generated python code is just a plain string, I would pickle that instead. You would have to ensure that when you load the pickled object that the string is then converted to python bytecode again

Given that the default setup is now casadi I will leave this up to a new issue