Addition of a new 3D model, model 18

[DimitriosBellos]

I modified model 8 into a new model (model 18). Now the 62 different ellipsoids are positioned also differently on the z axis, meaning that contrary to model 8 where all ellipsoids had z0 = 0.0, now in model 18 those have random z0 values. Care has been taken so that the ellipsoids are fully contained within the phantom area.

[dkazanc]

Many thanks Dimitrious. I'll add that one. Just FYI there is also a random generator which generates various spherical objects (random packing). They are mostly lie within a sphere of a certain diameter. May be this you could use as well. cheers