Allow larger tanks (change maximum from 200 to 300 cuft)

We don't handle doubles any different than single tanks - so while
200 cuft was a sane maximum size for a tank, once you dive with
doubles this logic fails.

We may or may not decide to implement special handling for doubles at some
point, but for now simply allow for tanks all the way up to double-150.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>