Add [XYZ]_CENTER conditionals

Marlin/Conditionals_post.h

   #endif
 
   /**
-   * Axis lengths
+   * Axis lengths and center
    */
   #define X_MAX_LENGTH (X_MAX_POS - (X_MIN_POS))
   #define Y_MAX_LENGTH (Y_MAX_POS - (Y_MIN_POS))
   #define Z_MAX_LENGTH (Z_MAX_POS - (Z_MIN_POS))
+  #define X_CENTER float((X_MIN_POS + X_MAX_POS) * 0.5)
+  #define Y_CENTER float((Y_MIN_POS + Y_MAX_POS) * 0.5)
+  #define Z_CENTER float((Z_MIN_POS + Z_MAX_POS) * 0.5)
 
   /**
    * CoreXY and CoreXZ