dd → datamodel Relation

File in include/ddl/dd	Includes file in include/ddl/dd/datamodel
dd.h	datamodel_datadefinition.h
dd_predefined_datatypes.h	datamodel_types.h
dd_predefined_units.h	datamodel_units.h
dd_struct_access.h	datamodel_datadefinition.h
ddstring.h	xml_datamodel.h