Did some research in the C++ FAQ and found out how to have vectors of objects that have no default constructor:  http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.5  Therefore, LinkGroup and CrossRefGroup were removed and the NifStream functions and << operators were shifted from blk_ref and IBlock to Link and CrossRef directly.  The Python will now just have to worry about inserting the owner into the constructor of all links.