title: Pandas - Propagate the string ending with certain substring down a column until a new one is listed
body: <p>I've converted a pdf table into a csv and need to clean up two index columns below.</p>
  <p>I'd like to propagate the string ending with &quot;LB&quot; down Column A until a new one is listed.</p>
  <p><strong>Trying to create two new columns, from this...</strong>:</p>
  <pre><code>A          B
  
  30,000 LB  NaN
  foo        high
  bar        low
  25,000 LB  NaN
  baz        high
  zoo        low
  </code></pre>
  <p><strong>...to this.</strong>:</p>
  <pre><code>A            B
  
  30,000 LB    high
  30,000 LB    low
  25,000 LB    high
  25,000 LB    low
  </code></pre>