title: drop group by number of occurrence
body: <p>Hi I want to delete the rows with the entries whose number of occurrence is smaller than a number, for example:</p>
  
  <pre><code>df = pd.DataFrame({'a': [1,2,3,2], 'b':[4,5,6,7], 'c':[0,1,3,2]})
  df
  </code></pre>
  
  <pre class="lang-none prettyprint-override"><code>   a  b  c
  0  1  4  0
  1  2  5  1
  2  3  6  3
  3  2  7  2
  </code></pre>
  
  <p>Here I want to delete all the rows if the number of occurrence in column 'a' is less than twice.<br>
  Wanted output:</p>
  
  <pre class="lang-none prettyprint-override"><code>   a  b  c
  1  2  5  1
  3  2  7  2
  </code></pre>
  
  <p>What I know: 
  we can find the number of occurrence by <code>condition = df['a'].value_counts() &lt; 2</code>, and it will give me something like:</p>
  
  <pre class="lang-none prettyprint-override"><code>2    False
  3    True
  1    True
  Name: a, dtype: int64
  </code></pre>
  
  <p>But I don't know how I should approach from here to delete the rows.<br>
  Thanks in advance!</p>