Added a test directory and a very simple first test
Added vim temporary backup files to the gitignore
I've slightly changed the interface of the model such that outside the module pepa_model users do no need to remember why and when to call pm.derive(). This is simply called by any procedure that requires the state space if the state space has not already been derived. This also fixes a slight bug that if the user does not specify any options the model's state space was nevertheless derived but then no results were produced, which obviously might be frustrating if the state-space is large
Whoops, should have taken away the commented out call to pm.derive() as this is obviously what source code control is for