title: Sum columns of two dataframes based on dataframes' id's
body: <p>I have these dataframes named test and rul. The rul dataframe contains constants in the RUL column which need to be added to the test's RUL based on the dataset and unit id. I have no clue how to achieve this and would be happy if anyone could lead me in the right direction.</p>
  
  <p>Thanks.</p>
  
  <p><code>
  Test:
  +-------+------------+---------+-----+
  |       | dataset_id | unit_id | RUL |
  +-------+------------+---------+-----+
  |     0 | FD001      |       1 |  30 |
  |     1 | FD001      |       1 |  29 |
  |     2 | FD001      |       1 |  28 |
  |     3 | FD001      |       1 |  27 |
  |     4 | FD001      |       1 |  26 |
  |   ... | ...        |     ... | ... |
  | 41209 | FD004      |     248 |   4 |
  | 41210 | FD004      |     248 |   3 |
  | 41211 | FD004      |     248 |   2 |
  | 41212 | FD004      |     248 |   1 |
  | 41213 | FD004      |     248 |   0 |
  +-------+------------+---------+-----+
  </code></p>
  
  <p><code>
  rul
  +-----+------------+---------+-----+
  |     | dataset_id | unit_id | RUL |
  +-----+------------+---------+-----+
  |   3 | FD004      |       4 |  75 |
  |  63 | FD003      |      64 |  28 |
  |  70 | FD002      |      71 | 148 |
  | 203 | FD002      |     204 | 121 |
  | 225 | FD004      |     226 |  88 |
  | ... | ...        |     ... | ... |
  |  45 | FD004      |      46 | 134 |
  |  60 | FD002      |      61 | 169 |
  | 140 | FD004      |     141 | 130 |
  |  94 | FD001      |      95 | 128 |
  | 161 | FD002      |     162 |  41 |
  +-----+------------+---------+-----+
  </code></p>