title: Multiply rolling window with an array (pandas)
body: <p>For example, I have a dataframe df1 consisting of 5 rows and an array x:</p>
  <pre><code>x=np.array([1, 2, 3, 4])
  
  Timestamps  A
  01:00       1
  02:00       2
  03:00       3
  04:00       4
  05:00       5
  </code></pre>
  <p>How can I multiple array x with df1.rolling(4) and get the sum of all products in each line? Seems like rolling command does not have the multiply function. My expected output:</p>
  <pre><code>Timestamps  A          
  01:00       NaN
  02:00       NaN
  03:00       NaN
  04:00       1*1 + 2*2 + 3*3 + 4*4 = 30
  05:00       2*1 + 3*2 + 4*3 + 5*4 = 40
  </code></pre>
  <p>Thank you so much for your help!</p>