Extrude and move arm at the same time, in time

Depends on #11 (closed)

• Send trajectory and monitor feedback
• Control extruder and motion simulataneously
• Extrude at fixed rate while moving
• Fix any timing issues (extrusion must start exactly/before arm movement)
• Execute multiple contours with extrusion
• Fix any extract/retract issues (these will need to be accounted for by the planner)
• Change to better extrusion method if not good enough
• Check extrusion over multiple layers
• Multiple layer print on multiple parts of the print bed (with both arms individually)