syncing niflib with xml (note: Union field missing from BoundingVolume; see niflib hack in docsys/nifxml.py)

Add helper method to ControllerSequence which allows for adding item by name and not with target attached.

Update UpdateTangentSpace to match nifskope and add my algorithm adapted from the Obsidian developers version.

   - Fix issue with exporting Skin data causing missing vertexes in nifskope
   - add helper methods for collision related export.
   - fix crashes when exporting animation

Add helper methods to transform shape and fix some typos.

Add miscellaneous collision support helper methods

It seems that a NIF file with NULL property links is still able to be opened by the game engine, so added tests for this in a few places rather than assuming that all links are not NULL.