title: Find missing values in a range across a partition, then forward fill values?
body: <p>I'm looking to find missing entries in a field, grouped by two columns as a partition, then forward fill the missing entries. </p>
  
  <p>Table I have: </p>
  
  <pre><code>England    Q1        1     Dogs
  England    Q1        2     Dogs
  England    Q1        3     Dogs
  Germany    Q1        1     Dogs
  Germany    Q2        1     Cats
  Germany    Q2        2     Cats
  France     Q1        1     Cats
  France     Q2        1     Cats
  </code></pre>
  
  <p>So let's say my range is 1 - 3, for every unique [Country][Quarter] combo, I want a full 1,2,3 in the [Number] column, with the [Animal] column forward filling the previous entry. </p>
  
  <p>Table I want: </p>
  
  <pre><code>England    Q1        1     Dogs
  England    Q1        2     Dogs
  England    Q1        3     Dogs
  Germany    Q1        1     Dogs
  Germany    Q1        2     Dogs
  Germany    Q1        3     Dogs
  Germany    Q2        1     Cats
  Germany    Q2        2     Cats
  Germany    Q2        3     Cats
  France     Q1        1     Cats
  France     Q1        2     Cats
  France     Q1        3     Cats
  France     Q2        1     Cats
  France     Q2        2     Cats
  France     Q2        3     Cats
  </code></pre>
  
  <p>Let me know if this doesn't make sense!</p>