different abbreviations for concrete and abstract types

[lucaferranti]

as far as I can tell, there is only one abbreviation TYPES for types templates. I think it might be useful to have separate templates for abstract and concrete types, e.g. one might want to do the following

@template TYPES = """
                $(TYPEDEF)

                $(DOCSTRING)

                ## Fields
                $(TYPEDFIELDS)
                """

@template ABSTRACTTYPES = """
               $(TYPEDEF)

               $(DOCSTRING)
              """

[MichaelHatherly]

Seems reasonable to me.