OSL camera coordinate systems #1

Sorry to horn in but I've seen some "no", "un", "dis" in some boolean methods of appleseed. Maybe it's a personal taste but I think it's more complicate to read codes that use such method when you need to "reverse" them (double negation): !toto.is_nonlinear() (is toto not non linear?) VS !toto.is_linear() (is toto not linear?).
I even wonder if I already talked about this with you Est in another pull request.
Once again, thanks a lot for your contributions Est! :)