Add simulation class

[Scottmar93]

Summary
Create a simulation class which wraps model, discretization, parameters etc.

Motivation
Can be extended to allow linking of the experiment, model, discretization, and solver together and allow each to reference each other. Also simplifies the pybamm user interface so that users only need to see what the actually are setting

[valentinsulzer]

See #39 and the branch issue-39-simulation-class - happy for you to take over on this though :)

[Scottmar93]

Ah nice thought you had deleted the branch so just made a quick mock up of what the class would look like. Will take a look at #39 as well. Doesn't seem like there is too much to do but might be nice to have some reset type features that would allow you to undiscretize your model etc.

[valentinsulzer]

Sounds good, note you can do disc.process_model(model, inplace=False) to leave the original model un-discretised