Enable absolute/relative extrusion option on non BBL printers (#205)

Add the option to use non-relative extrusion in printer config screen. Some extruders do not play well with relative extrusion...

Remove reference to multi-extruder

Remove reference to multi-extruder

change g-code comment for M83 relative extrusion

changed comment for M83

Remove commented line of code

In this specific case, we delete the old line of code because it refers to a variable that was deleted from the rest of the code (RELATIVE_E_AXIS)

src/libslic3r/GCode/CoolingBuffer.cpp

@@ -401,7 +401,7 @@ std::vector<PerExtruderAdjustments> CoolingBuffer::parse_layer_gcode(const std::
             }
             if ((line.type & CoolingLine::TYPE_G92) == 0) {
                 //BBS: G0, G1, G2, G3. Calculate the duration.
-                if (RELATIVE_E_AXIS)
+                if (m_config.use_relative_e_distances.value)
                     // Reset extruder accumulator.
                     current_pos[3] = 0.f;
                 float dif[4];