title: Pandas dataframe calculate average of n-columns for each row
body: <p>Given following <code>pandas</code> dataframe of <code>248 rows x 101 cols</code>, I need to calculate a new column containing the average of all the <code>sim-*</code> columns only (exclude the <code>Date</code> column).</p>
  <p><a href="https://i.stack.imgur.com/7rC24.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/7rC24.png" alt="enter image description here" /></a></p>