Archive materials class

[ioannaMitropoulou]

Archive materials and cleanup.

"""
hey @ioanna Mitropoulou something else: I was thinking it might make sense to also remove the MachineModel and the Material classes for now, since we are not really using them. In the example we are just calling them but then we are not using them.

--- Fabrication

robot_printer = RobotPrinter('UR5')
material_PLA = Material('PLA')
I think maybe in general we might not need 'Material' since I think often users want to set their own print speed, temperature etc. anyway and not use standard values, since robotic printing is not a standard process anyway. We can have some standard values but imo they don't have to rely on different materials.
Then as for robot models, I think for now our export is still general enough to be used by any kind of robot right? Sure we are focusing on UR's, but currently we are not yet changing something to make it only UR specific. I think at the current moment, compas_slicer is also more of a general slicing tool, which then allows the user to pick their favorite robot and command generation tool, right?
"""