Internalise pepa_model.derive() to pepa_model

This change started because if a user does:

    python pyPEPA.py <model>

without specifying any actions, then the state space is still derived, but nothing is done with it. If the model is rather large then obviously this is wasteful and a little frustrating for the user.

Getting rid of that behaviour non-trivial in pyPEPA.py is non-trivial. It stems from the fact that outside the module "pepa_model" the user needs to keep track themselves of whether or not the state space has been derived. Instead a better way is for the users to simply call pm.steady_state() etc and for those methods to derive the state space if it is required. Since the state space is stored within the PEPAModel class we can avoid recomputing the state space when it is not necessary.

I've also added a test directory, although it only contains one very simple model and no actual testing code.

regards,
allan