The behavior of na::clamp on vectors is strange.

See also the conversation #434.

It appears the `na::clamp` function is based on the comparison operators which themselves rely on the `PartialOrd` trait. However those comparison operators don't return a sensible value whenever the two vectors being compared are actually not comparable (`partial_cmp` returns `None`).

One solution to avoid confusion is to make it so `na::clamp` works only on scalars, and add a `.clamp` method to the vectors/matrices.