quat: rename Quat to Number
Created by: kortschak
This reduces stutter and parallels the convention in the proposed dual and hyperdual packages.
I was originally reluctant to do this, but I think the reduction of stutter and the now found mathworks page (also here, here and here) that uses "quaternion number" make it OK in my mind — though note that Hamilton named them "imaginaries" as a system.
Please take a look.