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/GCodeReader.cpp

@@ -82,7 +82,7 @@ const char* GCodeReader::parse_line_internal(const char *ptr, const char *end, G
         }
     }
     
-    if (gline.has(E) && RELATIVE_E_AXIS)
+    if (gline.has(E) && m_config.use_relative_e_distances)
         m_position[E] = 0;
 
     // Skip the rest of the line.