Implemented a least squares fit of the bed equation for auto bed leveling.

The code for the LSQ solver (qr_solve) is copyrighted by John Burkardt and released under LGPL here:
http://people.sc.fsu.edu/~%20jburkardt/c_src/qr_solve/qr_solve.html
(see qr_solve.cpp for further copyright information)

Marlin/Configuration.h

@@ -366,6 +366,15 @@ const bool Z_MAX_ENDSTOP_INVERTING = true; // set to true to invert the logic of
     
   #endif
   
+  // with accurate bed leveling, the bed is sampled in a ACCURATE_BED_LEVELING_POINTSxACCURATE_BED_LEVELING_POINTS grid and least squares solution is calculated
+  // Note: this feature occupies 10'206 byte
+  #define ACCURATE_BED_LEVELING
+  
+  #ifdef ACCURATE_BED_LEVELING
+     // I wouldn't see a reason to go above 3 (=9 probing points on the bed)
+    #define ACCURATE_BED_LEVELING_POINTS 2
+  #endif
+  
 #endif