title: Mutate taking a difference based on the criteria of different column
body: <pre>
  | weight    | Time     | Chick   | Diet    |
  |-----------|----------|---------|---------|
  |    42     |    0     |    1    |    1    |
  |    51     |    2     |    1    |    1    |
  |    59     |    4     |    1    |    1    |
  |    64     |    6     |    1    |    1    |
  |    76     |    8     |    1    |    1    |
  |    93     |    10    |    1    |    1    |
  |    106    |    12    |    1    |    1    |
  |    125    |    14    |    1    |    1    |
  |    149    |    16    |    1    |    1    |
  |    171    |    18    |    1    |    1    |
  |    199    |    20    |    1    |    1    |
  |    205    |    21    |    1    |    1    |
  |    40     |    0     |    2    |    1    |
  |    49     |    2     |    2    |    1    |
  </pre>
  
  <p>I need to take the current weight and subtract the weight at time 0. Time interval resets every 12th rows. Mutate it into a WeightGain column. Please help me do this in r.</p>