Ensemble support

[ptheywood]

The intial model (#2) will have been implemented as a single simulation (CUDASimulation).

As a stochastic simulation, running the simualtions many times would be required for useful scientific outputs.

FALME GPU includes an interface to do this, with opportunity for performance improvements (and/or simplified running of the model).

This will likely require CLI change(s), and some refactoring.

The same total simulations would be possible without this, but the ensemble interface may offer improved performance and/or simplify running simulation batches.

[ptheywood]

Switched to agent initialisation via an init function in #24, which is a pre-requisite of ensemble support.

CLI changes still needed, and anonymous namespace members used to store data between simulations will need adjusting to do so in a thread-safe fashion.