title: How to round datetime64 values
body: <p>I have a numpy array of <code>datetime64</code>, and I would like to round off the sub-second values of the array elements. E.g., from <code>2001-1-1 10:33:32.5</code> to <code>2001-1-1 10:33:32.0</code>. I am looking for a vecotrized method.</p>
  
  <p>More generally, I am looking for a vectorized method to round to any frequency (minutes, days, etc.).</p>