reading grads file (except "data type station" or "grib")
from the grads control file even if there is multiple data files.
keyword GLAMBOUNDARY (via computegrid.pro) a 2 elements vector,
{lon1,lon2], giving the longitude boundaries that should be
used to visualize the data.
lon2 > lon1
lon2 - lon1 eq 360
key_shift will be automatically defined according to GLAMBOUNDARY.
keyword IODIRECTORY
a string giving the name of iodirectory
(see isafile.pro for all possibilities).
default value is common variable iodir