title: Divide one numpy array by another only where both arrays are non-zero
body: <p>What's the easiest, most Pythonic way to divide one numpy array by another (of the same shape, element-wise) only where both arrays are non-zero?</p>
  
  <p>Where either the divisor or dividend is zero, the corresponding element in the output array should be zero.  (This is the default output when the divisor is zero, but np.nan is the default output when the dividend is zero.)</p>