Explain the concept of units and how they represent a real rolling mill

[RichardPfr]

We should add a section or extend an existing one to better explain how the units PyRolL uses work and how they represent a real rolling mill. The question "How do i get from a rolling mill to our units?" should be answered.

This might be best done graphically - for example by showing a sideview of a rolling mill and mark all units and PyRolL specific subparts in it.

A similar graphic can already be found here.

This graphic could also be added to the explanations on the startup page, as understanding this concept is very useful when one wants to work with PyRolL.