Adds support for multi extrusion print based on color flag in the STL

- this feature must be enabled by the '-e' flag
- what this does, is that it groups all faces with same color into same
  SimpleVolume. Model then has as many volumes, as there were distinct
colors for the faces, so it requires you to color your model correctly.
The effect of grouping the faces is that at the end multiple extruders
will be used.
- the volumes get sorted by the (numerical) value of the color (so color
  1 will be printed with extruder 1 and volume with color 2 with
extruder 2 no matter what was the order of faces discovered in the STL
file