Issue 658 experiment

[valentinsulzer]

Description

Added experiment class and integration with the simulation class
Fixes #658

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 #807 into master will increase coverage by <.01%.
The diff coverage is 97.2%.

@@            Coverage Diff             @@
##           master     #807      +/-   ##
==========================================
+ Coverage    98.2%   98.21%   +<.01%     
==========================================
  Files         180      181       +1     
  Lines       10110    10405     +295     
==========================================
+ Hits         9929    10219     +290     
- Misses        181      186       +5

Impacted Files	Coverage Δ
pybamm/expression_tree/binary_operators.py	93.41% <ø> (ø)	⬆️
pybamm/parameters/parameter_values.py	100% <100%> (ø)	⬆️
pybamm/solvers/casadi_solver.py	100% <100%> (ø)	⬆️
...m/models/full_battery_models/base_battery_model.py	100% <100%> (ø)	⬆️
pybamm/solvers/idaklu_solver.py	89.21% <100%> (ø)	⬆️
...ubmodels/electrolyte/base_electrolyte_diffusion.py	100% <100%> (ø)	⬆️
pybamm/solvers/scikits_ode_solver.py	85.48% <100%> (ø)	⬆️
pybamm/models/submodels/base_submodel.py	92.64% <100%> (ø)	⬆️
pybamm/models/submodels/particle/base_particle.py	97.43% <100%> (+0.13%)	⬆️
...odels/full_battery_models/lithium_ion/basic_dfn.py	100% <100%> (ø)	⬆️
... and 24 more

---

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 3c6dda7...0a3517f. Read the comment docs.

[rtimms]

this looks great thanks @tinosulzer ! very user-friendly input for the experiments. it might be good if we could add some documentation detailing the structure of the strings and explicitly highlighting the different keywords that can be used