title: Calculating mean of each row, ignoring 0 values in python
body: <p>I have a data frame with 1000 rows and 10 columns.<br>
  3 of these columns are 'total_2013', 'total_2014' and 'total_2015'
  I would like to create a new column, containing the average of total over these 3 years for each row, but ignoring any 0 values. </p>