Add keyvalue to comp_propcombine_volume to allow combine negation

[ElliotElison]

Might be useful to be able to place a negative combine volume in case you don't want to props with a particular model in a particular spot to be combined, while you do want them to be combined elsewhere. Adding the prop to the propcombine blacklist will cause it to not be combined anywhere.

[TeamSpen210]

I'm not really sure how common situations like that would be?