Real-time filament diameter measurement and control

This feature allows the printer to read the filament diameter
automatically and adjust the printer in real time.  Added code to read
an analog voltage that represents a filament diameter measurement.  This
measurement is delayed in a ring buffer to compensate for sensors that
are a distance away from the extruder.  The measurement is used to
adjust the volumetric_multiplier for the extruder.  Some additional g
codes (M404, M405, M406, M407) are used to set parameters and turn
on/off the control. g code M221 is updated.  Pins for RAMPS1.4, RAMBO,
and Printrboard are identified for analog input.  The configuration file
is updated with relevant user parameters.

Marlin/temperature.h

@@ -31,6 +31,14 @@
 void tp_init();  //initialize the heating
 void manage_heater(); //it is critical that this is called periodically.
 
+#ifdef FILAMENT_SENSOR
+// For converting raw Filament Width to milimeters 
+ float analog2widthFil(); 
+ 
+// For converting raw Filament Width to an extrusion ratio 
+ int widthFil_to_size_ratio();
+#endif
+
 // low level conversion routines
 // do not use these routines and variables outside of temperature.cpp
 extern int target_temperature[EXTRUDERS];