title: Concatenating strings across two rows in pandas dataframes
body: <p>I have a table like so after performing some data scraping on a pdf:</p>
  <pre><code>index colA           colB                     colC colD colE colF colG
  -------------------------------------------------------------------
   1    ABCD  veryLongTextThatShouldNotCutOff   12   x    x    x    x
   2    ABCD  veryLongText                      14   x    x    x    x
   4    NaN   ThatShouldNotCutOff               NaN  NaN  NaN  NaN  NaN
   5    ABCD  veryLongTextThatShouldNotCutOff   12   x    x    x    x
   6    ABCD  veryLongTextThatShouldNotCutOff   x    19   x    x    x
   7    ABCD  veryLongText                      21   x    x    x    x
   9    NaN   ThatShouldNotCutOff               NaN  NaN  NaN  NaN  NaN
   10   ABCD  veryLongTextThatShouldNotCutOff   13   x    x    x    x
  ...
  </code></pre>
  <p>I need to clean the data up so that information in rows with index 2 and 4 are merged together like so, and with the indexes in the right order again.</p>
  <pre><code>index colA           colB                     colC colD colE colF colG
  -------------------------------------------------------------------
   1    ABCD  veryLongTextThatShouldNotCutOff   12   x    x    x    x
   2    ABCD  veryLongTextThatShouldNotCutOff   14   x    x    x    x
   3    ABCD  veryLongTextThatShouldNotCutOff   12   x    x    x    x
   4    ABCD  veryLongTextThatShouldNotCutOff   x    19   x    x    x
   5    ABCD  veryLongTextThatShouldNotCutOff   21   x    x    x    x
   6    ABCD  veryLongTextThatShouldNotCutOff   13   x    x    x    x
  ...
  </code></pre>
  <p>Essentially, some rows with colA as NaN needs to be &quot;concatenated&quot; with the previous rows.</p>
  <p>How do I do it using pandas?</p>