title: Pandas ffill and bfill in the same column - conditional
body: <p>I have a column with chemistries and at some point I have the change of them. I want to ffill until the &quot;changepoint&quot; and bfill after it.</p>
  <p>I have several change points like this:</p>
  <pre><code>date0   NaN
  date1   chem1
  date2   NaN
  date3   NaN
  date4   change
  date5   NaN
  date6   chem2
  date7   chem2
  date8   NaN
  date9   change
  date10   NaN
  date11   chem3
  date12   NaN
  </code></pre>
  <p>I would like to have it:</p>
  <pre><code>date0   chem1
  date1   chem1
  date2   chem1
  date3   chem1
  date4   change
  date5   chem2
  date6   chem2
  date7   chem2
  date8   chem2
  date9   change
  date10   chem3
  date11   chem3
  date12   chem3
  </code></pre>
  <p>any suggestions on the best approach here?</p>