The /FORMAT option governs how the data part (pixel values) of the
RGDATA file must be read by GreG. fmt is a valid fortran format (default
is 10z8, the minimal way to encode real*4 data). code may be BY , R4 ,
R8 , I4 , I2, depending if the data written in the file are BYtes,
Real4, Real8, Integer4 or Integer2. (the values in the GreG map will
always be Reals*4 after the read is done).