Add `Shear` unit

[ChRen95]

For correct representation of a rolling mill, a ``Shearunit should be added. The unit can be derived from theTransport` class and should feature a label as well as a `cut length` or however this is called.

[ChRen95]

So I started adding the unit, due to the iterative nature of PyRolL, I encountered a problem when changing the length of the material as it is "chopped" to 0 due to the iterative nature of PyRolL. Currently I can't think of a solution @axtimhaus any suggestion?