Generating core-shell spheres

This code generates a 3d arrangement of core-shell configuration of spheres.

User inputs x,y and z values, which are the dimensions of the arrangement of core-shell spheres in x,y and z directions respectively. In the screen-shot provided, you can see a 3x3x3 arrangement of core-shell spheres. Similarly, you can give any value for x,y and z to generate your own arrangement of spheres. Displacement between the spheres can be varied by making minor changes to the code.