Create units for `qx`, `qy` and `qz`

[kif]

Defining these 3 units will allow to perform rebinning in x/y or x/z or y/z planes.

How to define these: Come back to the definition:

vect(q) = vect(k_in) - vect(k_out)

k_in = 2*pi/lambda along -z k_out has the same modulus but is along the line from origin to the given pixel and can be project on axis x,y and z.

Requested by ID10