Consider separating "forcing" from explicit-Rep

[chiphogg]

I have always conflated these ideas, probably because static_cast does both. But it could be nice to be able to write, say, inches(50).force_as(feet): we wouldn't need to repeat the Rep, and the "force" makes the potential lossiness more obvious. This would free up q.as<T>(unit) to respect the safety checks.

[chiphogg]

On further reflection and discussion, I'm leaning towards coerce_as rather than force_as, since "force" already has a different meaning in units-land.