title: Convert pandas.Series from dtype object to float, and errors to nans
body: <p>Consider the following situation:</p>
  
  <pre><code>In [2]: a = pd.Series([1,2,3,4,'.'])
  
  In [3]: a
  Out[3]: 
  0    1
  1    2
  2    3
  3    4
  4    .
  dtype: object
  
  In [8]: a.astype('float64', raise_on_error = False)
  Out[8]: 
  0    1
  1    2
  2    3
  3    4
  4    .
  dtype: object
  </code></pre>
  
  <p>I would have expected an option that allows conversion while turning erroneous values (such as that <code>.</code>) to <code>NaN</code>s. Is there a way to achieve this?</p>