Remove hard-coding of arrivals data file from Scenario

[TomMonks]

At the moment Scenario has the following logic to initialise time dependent arrivals sampling:

class Scenario:
    
   # Other methods definitions

    def init_nspp(self):
        
        # read arrival profile
        self.arrivals = pd.read_csv(NSPP_PATH)
        self.arrivals['mean_iat'] = 60 / self.arrivals['arrival_rate']
       
        # maximum arrival rate (smallest time between arrivals)
        self.lambda_max = self.arrivals['arrival_rate'].max()
        
        # thinning exponential
        self.arrival_dist = Exponential(60.0 / self.lambda_max,
                                        random_seed=self.seeds[8])
        
        # thinning uniform rng
        self.thinning_rng = Uniform(low=0.0, high=1.0, 
                                    random_seed=self.seeds[9])

The constant NSPP links to the data file containing the arrival profile. This limits experimentation and testing. A solution is to rewrite the function to use a member attribute of the class e.g. self.nspp_path that is set during __init__ where NSPP is the default value.