Velocity has no 3-D definition

[louis-langholtz]

Expected/Desired Behavior or Experience:

There is a playrho::d3::Velocity type defined like there is for playrho::d2::Velocity.

Actual Behavior:

There is not currently a definition for a playrho::d3::Velocity type.

Note that this is far from the only structure that's needed in playrho::d3, it's just one of many that will be needed for the 3-D Support project and I'd prefer for each of the needed structures/files to have their own issue so the project can be more incrementally worked on.