use GRIB ltype to define GENERIC zaxis

ChangeLog

prog/cdi.c

src/cdi.h

src/stream_cdf.c

src/stream_ext.c

src/stream_grb.c

src/stream_ieg.c

src/stream_srv.c

src/taxis.c

src/varscan.c

src/varscan.h

src/vlist.c

src/zaxis.c