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).