title: Fill Nan with all the information from previous week
body: <p>I have a dataframe that looks like:</p>
  <pre><code>    Week  Store End Cap       UPC
  0      1      1       A  123456.0
  1      1      1       B  789456.0
  2      1      1       B  546879.0
  3      1      1       C  423156.0
  4      1      2       A  231567.0
  5      1      2       B  456123.0
  6      1      2       D  689741.0
  7      2      1       A  321654.0
  8      2      1       B       NaN
  9      2      1       C  852634.0
  </code></pre>
  <p>I want for every row where I have a Nan UPC to go and check the previous week, match on Store and End Cap and then grab <strong>all</strong> the information from previous week where we are matching.</p>
  <p>So in the above example (2/1/B) would match with both the second and third row that show (1/1/B) and the desired output would look like this:</p>
  <pre><code>       Week  Store End Cap       UPC
      0      1      1       A  123456.0
      1      1      1       B  789456.0
      2      1      1       B  546879.0
      3      1      1       C  423156.0
      4      1      2       A  231567.0
      5      1      2       B  456123.0
      6      1      2       D  689741.0
      7      2      1       A  321654.0
      8      2      1       B  789456.0
      9      2      1       B  546879.0
     10      2      1       C  852634.0
  </code></pre>
  <p>We now have <strong>both</strong> 789456 and 546879 show up for (2/1/B)</p>
  <p>How can I go around doing this?</p>
  <p>I tried sorting and forward filling but that only gets me 1 of the values not all.</p>