title: floor and ceil with number of decimals
body: <p>I need to floor a float number with an specific number of decimals.</p>
  <p>So:</p>
  <pre><code>2.1235 with 2 decimals --&gt; 2.12
  2.1276 with 2 decimals --&gt; 2.12  (round would give 2.13 which is not what I need)
  </code></pre>
  <p>The function <code>np.round</code> accepts a <code>decimals</code> parameter but it appears that the functions <code>ceil</code> and <code>floor</code> don't accept a number of decimals and always return  a number with zero decimals.</p>
  <p>Of course I can multiply the number by <code>10^ndecimals</code>, then apply floor and finally divide by <code>10^ndecimals</code></p>
  <pre><code>new_value = np.floor(old_value * 10**ndecimals) / 10**ndecimals
  </code></pre>
  <p>But I'm wondering if there's a built-in function that does this without having to do the operations.</p>