parallelize computation

[aligator]

There are some possibilities for parallelization using go routines:

• the slicer can parallelize the slice process for each layer
• the slicer can also parallelize the makePolygons and GenerateLayerParts per layer
• the modifier processes each modifier after each other, but some modifiers itself could parallelize the execution per layer, if they only use and modify data from one layer.
• maybe the gcode generation could also be parallelized per layer and then the resulting gcode parts could be combined at the end.