Add an Experiment class

[Scottmar93]

Summary
Add an Experiment class where the user can define all thing related to the specific set up of an experiment, including:

• initial conditions
• cycling profile
• SOC to concentrations calculator
• cooling set up?

Feel free to suggest other things that should be in this

Motivation
Performing different experiments on models will be one of the main use cases of PyBaMM so we should have a clear and simple method for setting this up.

[brosaplanella]

Some experiments (GITT, HPPC...) are based on applying pulses to the battery. This can be easily implemented as a time dependent current, but it would be interesting to include a pulse function so it is friendly to users who are not used to coding. This can be helpful when including it in the GUI.