Runtime class

[TomNicholas]

I would like to see a more abstract representation of the system on which we are running CStar.

I'm not sure exactly what form this should take, but I think the scientific configuration should be specified as separately as possible from the specifics of the HPC system.

One idea in this direction is to have a class to represent the "runtime" or "executor", which is passed to the case.run() method, and contains all information about the machine. However if we also need to use machine-specific arguments during compilation then I guess we have to pass the machine information in earlier too.