Unify thickness (and other properties?) between layers and structures.

[EmCeBeh]

It might be useful to get the thicknesses a structures, not knowing weather it is a layers or again a structure.

However, this is not as trivial as:

• layers have the attribute thickness, and NO function get_thickness()
• structures have NO attribute thickness, but the function get_thickness()

Is this intended?
Can it be generalised, such that "any" object, which has a thickness has the same way of accessing it?

Maybe this might be usefull for other properties, too?

Thanks

Martin & @S0nn1ck

[dschick]

indeed one could unify it for common properties. Could you list all of them, which come into your mind?
thickness, mass are straightforward. Other ones need to be averaged and might not be too useful anyway.

I welcome any pull request for this improvement.