Compilation and simulations class

[GJHSimmons]

We discussed that the current system class currently plays two roles:

1. Using data in a population, it creates a system of ODEs ready for numerical simulation.
2. It creates a simulation according to given parameters (currently end_time, n_steps, mode).

These two functionalities should be separated out. The first should create a system object associated to a population (potentially compiled from an internal call of population, for example population.compile().

The second should create one or more simulation classes according to different simulation parameters. A container subclass simulations will then be created to hold multiple different simulations (which could either come from the same system or across systems). This class would contain functionality to compare the simulations it contains.