title: Select rows containing certain values from pandas dataframe
body: <p>I have a pandas dataframe whose entries are all strings:</p>
  
  <pre><code>   A     B      C
  1 apple  banana pear
  2 pear   pear   apple
  3 banana pear   pear
  4 apple  apple  pear
  </code></pre>
  
  <p>etc. I want to select all the rows that contain a certain string, say, 'banana'. I don't know which column it will appear in each time. Of course, I can write a for loop and iterate over all rows. But is there an easier or faster way to do this?</p>