Given a value of an interval, this function returns the 
        next interval value

def next_interval(self, interval):
        """
        Given a value of an interval, this function returns the 
        next interval value
        """
        index = np.where(self.intervals == interval)
        if index[0][0] + 1 < len(self.intervals):
            return self.intervals[index[0][0] + 1]
        else:
            raise IndexError("Ran out of intervals!")