Saving progress before adding support for Steven's wrfout_trop files
Initial implementation of code to support Steven's wrf_tropo files on a
limited area domain. Code is buggy since I'm waiting to hear back about
what U,V mean in the output files.

The limited area grid means that we need to handle the case in the advection
correspondence where points leave the grid. Currently, we just ignore
points that advect to near the edge of the domain.
Rotating horizontal winds in file with cosalpha,sinalpha that must be
the same for all files and times as written.
Some obvious bug fixes too.
LAM edits. Fixed divide by 0 if entire basin advected out of domain.
Remember that we're still not finding closest cell to point properly.