Add Scale() method

[Zyl9393]

I have use for a method which takes a vector and returns a new vector where every element is the product of the respective elements of the vector and the elements of the receiver vector, e.g.:

func (v1 Vec3) Scale(v2 Vec3) Vec3{
    return Vec3{v1[0] * v2[0], v1[1] * v2[1], v1[2] * v2[2]}
}

In my case I use it to scale the vertices of a 3D object with a per-axis-controllable factor, which is a common CGI feature to have to provide more artistic control over having just a single factor that gets used for each dimension.

It is trivially implementable and testable. This issue is mostly to discuss method name and whether this is wanted. I'm open to creating an MR if this is ok.

[luisgargitter]

Does a Matrix not do the trick?

v := Diag3(scale).Mul3x1(v)