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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def minimum_image_dr( self, r1, r2, cutoff=None ):
 """ Calculate the shortest distance between two points in the cell, accounting for periodic boundary conditions. Args: r1 (np.array):
 fractional coordinates of point r1. r2 (np.array):
 fractional coordinates of point r2. cutoff (:obj: `float`, optional):
 if set, return zero if the minimum distance is greater than `cutoff`. Defaults to None. Returns: (float):
 The distance between r1 and r2. """

        delta_r_vector = self.minimum_image( r1, r2 )
        return( self.dr( np.zeros( 3 ), delta_r_vector, cutoff ) )