Increase AP on critical wounds

From a user report:

> One of the Tyranids detachments has an Enhancement that gives this ability:
> 
> "On a Critical Wound, improve the Armour Penetration characteristic of that attack by 1."
> 
> Necrons' Canoptek Acanthrites have a similar ability, and T'au Empire's Cadre Fireblades have a -3 AP version. I'm not sure about anyone else. 
