If flow is defined by spd/dir, always color by speed.
Don't adjust the longitudes if it's a TrivialMapProjection.  Fixes one
or two things, will probably break something else.
nc 4.3 doesn't use DiskCache for the grib indexing, it now uses
DiskCache2 (go figure).  Fix the problem of the grib index files being
written with the files instead of in the cache directory.
Adjust a bit for short/wide projections.  Really just a hack