Added a new RelatedPackageType for specifying a relationship to a Package.
Added a new RelatedPackagesType containing a list of Related_Package elements each of RelatedPackageType.
Added an optional Related_Packages element to STIXType
Added new RelatedPackageRefType
Added new RelatedPackageRefsType
Added new optional Related_Packages element to each STIX core construct type
merge
Corrected @idref annotation on RelatedPackageRefType

It is allowed for the RelatedPackageRefType to contain content with an
@idref (unlike the rest of STIX) because the @idref is a level up from
where it normally is.