Feature/task 20 add testcases with spe

[ghost]

Added test cases for None and Geometry shapes in Im- and Exporter:

• Geometry now updates the stl file when it get's exported
• None shape now creates a dummy part instead of not having a part file
• Updated shape handling in JsonPartFactory (now get's shape from the sheet)

[ghost]

Closes https://github.com/virtualsatellite/VirtualSatellite4-FreeCAD-mod/issues/20