Re-calculate extrusion for Ulti-gcode

[peteruithoven]

Ulti-gcode (used by Ultimaker 2 type machines) works with cubic mm, instead of mm. We should re-calculate the Retraction amount.
See: #23 (comment)

[Ghostkeeper]

2.3 certainly won't see a fix for this, but I've put this in the list for 2.4 so when we start with 2.4 we'll decide when to fix this.

[Ghostkeeper]

Our product manager just stopped this because it has been inactive for a few months.