Add points.

This adds the Pnt{1,2,3,4,5,6} structures.
This adds the traits:
  − AnyPnt
  − FloatPnt
  − PntExt
  − FloatPntExt
  − Orig (to return the zero point)
This adds operator overloading:
  − Pnt + Vec
  − Pnt - Vec
  − Pnt * Scalar
  − Pnt / Scalar
  − Pnt + Scalar
  − Pnt - Scalar
  − Iso * Pnt
  − Rot * Pnt
  − Pnt * Iso
  − Pnt * Rot
This changes some behavior:
  − Iso multiplication with a Vec does not translate the vector any more.
  − ToHomogeneous adds a 0.0 at the end of a Vec and a 1.0 at the end of a Pnt.
  − FromHomogeneous performs w-normalization on a Pnt, but not on a Vec.
  − The Translate<Vec> trait is never implemented (i-e. a Vec is not to be translated).

cc #25