Add mesh

[fannyzotter]

Sometimes a fine mesh is used for cooling in a beamline.
I think it could be implemented relatively easy as a variation of Slit.
params would be
-heitht
-width
-bar width
-opening width( we could only do squares)

With some collision detection optimisation Sven sugested.

Should we keep fraunhofer diffraction for that?

[Atraxus]

What does such a mesh actually look like? Based on the parameters, it seems more like a specific type of slit.