title: Pandas round to the nearest "n"
body: <p>Numeric series have a nice rounding method for rounding to powers of ten, eg</p>
  
  <pre><code>&gt;&gt;&gt; pd.Series([11,16,21]).round(-1)
  0    10
  1    20
  2    20
  dtype: int64
  </code></pre>
  
  <p>Is there an equivalently nice syntax for rounding to the nearest 5 (or other non-power of 10)? I'm sort of wishing that <code>round</code> could take non-integer values?</p>