Implement RCN extra data flag

A common pattern in animated meshes is to use a node structure like the following which throws Root Collision Node warnings. While this is largely a pattern I believe used for the vanilla editor many OpenMW players will encounter this in modded setups and at least 1 vanilla mesh uses this pattern.

-NiStringExtraData MRK
   -NiStringExtraData RCN

I did read somewhere that RCN isn't used at all in OpenMW so potentially this issue should be amended but I don't know the full scope of the issue.

Edit by AnyOldName3: code block around NIF structure so the Markdown interpreter didn't cram it all onto one line.