Retain offset in BoundingBox::contract and expand

[Minikloon]

This is a breaking change to those methods but I expected those to behave like this so much that I think it's a bug.

[mworzala]

This is a really 'sneaky' breaking change and would be frustrating to debug. I think there should simply be some new methods added to work this way and javadocs added explaining the difference rather than breaking here.

[Minikloon]

I would call them expandInPlace but I think they're pollution.
I'll just
boundingBox.expand(1, 1, 1).withOffset(boundingBox.relativeStart())
forever