Prime/wipe tower
Instructs IceSL to build a prime/wipe tower structure 
alongside the print. The tower is used to wipe and prime
the extruders when switching material in multi-material
prints. The tower only prints when required (it will not
appear on single material prints).
The tower is either automatically placed, or its position
is controlled by variables 'extruder_swap_location_x_mm'
and 'extruder_swap_location_y_mm' when a swap 
location is specified in the printer scripts, and 
'extruder_swap_at_location' is enabled.
Note that the tower is added even on single material prints,
for instance to allow for cooling between layers when 
printing parts with a small cross-section (see also
enable_min_layer_time)