fixed a bug in the complex shape helper class used for exporting or importing. now it properly creates skin partitions.

Added support for adding new control points and editing the offsets in the NiBSplineTransformInterpolator for the start of the control points. Also made niflib compile with the latest version of qhull.

Made various NiSkinPartition methods public, so they can be manipulated (contributed by gentle_sal, see issue #3411675).

Added method to remove match data from NiTriShapeData (contributed by gentle_sal, see issue #3411674).

Added subshape and material access to bhkConvexListShape (contributed by gentle_sal, see issue #3411666).

Use single root for nif in WriteNifTree and ResolveMissingLinkStack, to make interface consistent with the rest of niflib.

Add first vertex in match group to the head of the group, so indices are consequetive (might have a small performance benefit, and looks more like official nifs).