This function writes the field metadata for a grid field not defined by the Grid API.
Result = EOS_GD_WRITEFIELDMETA(gridID, fieldname, dimlist, numbertype)
Returns SUCCEED(0) if successful and FAIL(-1) otherwise
Grid id (long) returned by EOS_GD_CREATE or EOS_GD_ATTACH.
Name of field (string) that metadata information is to be written.
Dimension list of field (long).
Number type of data in field (long).
status = EOS_GD_writefieldmeta(gridID, "ExternField", $ "Ydim,Xdim", 5)