title: get mean for all columns in a dataframe and create a new dataframe
body: <p>I have a  dataframe with only numeric values and I want to calculate the mean for every column and create a new dataframe.<br>
  The original dataframe is indexed by a datetimefield. The new dataframe should be indexed by the same field as original dataframe with a value equal to last row index of original dataframe.</p>
  
  <p>Code so far</p>
  
  <pre><code>mean_series=df.mean()     
  df_mean= pd.DataFrame(stddev_series)
  df_mean.rename(columns=lambda x: 'std_dev_'+ x, inplace=True)
  </code></pre>
  
  <p>but this gives an error</p>
  
  <pre><code>df_mean.rename(columns=lambda x: 'std_mean_'+ x, inplace=True)
  TypeError: ufunc 'add' did not contain a loop with signature matching types dtype('S21') dtype('S21') dtype('S21')
  </code></pre>