isMoveAndExtrude

True iff any of x,y,z have changed and e has increased. i.e. the head has moved, and we have extruded filament by a positive amount. This will be false for simple movements without extrusions. Also false for extrusions when the head doesn't move (such as feeding filament back after a retraction) Also false for a retraction, regardless of whether the head moved during the retraction. NOTE, I have assumed that undoing a retraction (i.e. a positive extrusion amount corresponding with the negative extrusion amount of the retraction) will take place while the head is stationary. If this assumption is wrong, and undoing a retraction also moves the head, then findHeights() will not work correctly.