Add reference to GDP trajectories

As suggested [here](https://github.com/iiasa/message_doc/pull/27#pullrequestreview-273586234) in #27:

> We may want to refer to the original GDP trajectories (Dellink probably?) and just say that we supplied these trajectories to calibration [routines].

Review PR #27 and the linked review comment first, focusing on the requested reference to the original GDP trajectories and their use in calibration routines. The documentation is done when it identifies the original trajectories and clearly states that they were supplied to calibration routines.

Written by the indexing model from the issue text.