H5Tinsert adds another member to the compound datatype
type_id. The new member has a name which must be unique within
the compound datatype. The
offset argument defines the start
of the member in an instance of the compound datatype, and
field_id is the datatype identifier of the new member.
http://www.hdfgroup.org/HDF5/doc/RM/RM_H5T.html#Datatype-Insert