Made it possible to delay telling a Link and CrossRef which block owns them until the point where they store an actual link or cross reference.

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.

Created Link, LinkGroup, CrossRef, and CrossRefGroup classes which automate the process of adding and removing links and cross references to a block.  Had to put them in Nif_Blocks to avoid including that file in NIF_IO.

Updated MergeNifTrees to handle some cross references (skin blocks).  Other blocks with cross references can be added by overloading the ABlock::ReassignCrossRefs function.

Added documentation to IControllerSequence.

Fixed ReadString function so that it can read strings with \0 characters in them.

* Support for all ten NiStringPalette offsets.
* There's still a problem with NiStringPalette: apparently the C++ string type does not allow null characters? ... Maybe we can try to find a workaround in the string reader function.

* Changed the radius algorithm.

Now the calculated center & radius values should correspond to the values found in official NIF files.

Brought NiControllerSequence up to date with DocSys, but commented out interface functions for later implementation.

Block implementation is current with DocSys again.

Small changes to constness of some functions.

Simplified the ITriShapeData and ITriStripsData interfaces slightly.

* fixed skeleton root calculation