Converts fractional coordinates of trajectory into positions

def to_positions(self):
        """
        Converts fractional coordinates of trajectory into positions
        """
        if self.coords_are_displacement:
            cumulative_displacements = np.cumsum(self.frac_coords, axis=0)
            positions = self.base_positions + cumulative_displacements
            self.frac_coords = positions
            self.coords_are_displacement = False
        return