Final release for GMD publication

This release includes all changes that are represented in the 2021 GMD publication "Performance of offline passive tracer advection in the Regional Ocean Modeling System (ROMS; v3.6, revision 904)" by Kristen M. Thyng, Daijiro Kobashi, Veronica Ruiz-Xomchuk, Lixin Qu, Xu Chen, and Robert D. Hetland.

Offline passive tracer in ROMS

This contains all of the COAWST code to run the simulation. Files have been changed in the ROMS directory to allow for offline passive tracer advection, as described in a manuscript.