title: Conditional forward fill between integer values
body: <p>I have a dataframe column of integer values, with empty NaNs between them. i want to forward fill between the start of a new integer and the last instance of that same integer. </p>
  
  <pre><code>0   NaN
  1   2.0
  2   NaN
  3   NaN
  4   2.0
  5   NaN
  6   NaN
  7   3.0
  8   NaN
  9   NaN
  10  NaN
  11  3.0
  12  NaN
  13  NaN
  14  4.0
  15  NaN
  16  4.0
  </code></pre>
  
  <p>So my resultant dataframe would look like this below. How would i do this? My first approach was using enumerate and conditional statements with isinstance(x,int).</p>
  
  <pre><code>0   NaN
  1   2.0
  2   2.0
  3   2.0
  4   2.0
  5   NaN
  6   NaN
  7   3.0
  8   3.0
  9   3.0
  10  3.0
  11  3.0
  12  NaN
  13  NaN
  14  4.0
  15  4.0
  16  4.0
  </code></pre>