fix: z_distance_top/bottom was computed in micron rather than in layers (CURA-1013)
fix: z_distance_top/bottom was computed in micron rather than in layers (CURA-1013)

Ultimaker/CuraEngine,alephobjects/CuraEngine,totalretribution/CuraEngine,ROBO3D/CuraEngine,totalretribution/CuraEngine,totalretribution/CuraEngine,ROBO3D/CuraEngine,alephobjects/CuraEngine,Ultimaker/CuraEngine,ROBO3D/CuraEngine,alephobjects/CuraEngine