Write river locations into river forcing file rather than grid file

[NoraLoose]

Currently, ROMS-Tools writes the river locations into the grid file because this is what ROMS expects us to do. However, this is causing tons of issues. @dafyddstephenson is working on changes upstream (in the ROMS source code), see CESR-lab/ucla-roms#52. With these changes, we can write the river locations into the river forcing file. This should solve issue #226.