Ported over Johann Rocholl's improvements for delta printers:

- Nonlinear auto bed leveling code (includes G29, G30, Z_RAISE_AFTER_PROBING). Cleaned it up to be a delta-specific AUTO_BED_LEVELING_GRID code path.
- Allen key z-probe deployment and retraction code. Cleaned it up and added safety checks.

Marlin/example_configurations/delta/Configuration_adv.h

@@ -455,9 +455,27 @@ const unsigned int dropsegments=5; //everything with less than this number of st
 //===========================================================================
 
 #if defined (ENABLE_AUTO_BED_LEVELING) && defined (DELTA)
-  #error "Bed Auto Leveling is still not compatible with Delta Kinematics."
+
+  #if not defined(AUTO_BED_LEVELING_GRID)
+    #error "Only Grid Bed Auto Leveling is supported on Deltas."
+  #endif
+  
+  #if defined(Z_PROBE_SLED)
+    #error "You cannot use Z_PROBE_SLED together with DELTA."
+  #endif
+
+  #if defined(Z_PROBE_REPEATABILITY_TEST)
+    #error "Z-probe repeatability test is not supported on Deltas yet."
+  #endif
+
 #endif  
 
+#if defined(Z_PROBE_ALLEN_KEY)
+  #if !defined(AUTO_BED_LEVELING_GRID) || !defined(DELTA)
+    #error "Invalid use of Z_PROBE_ALLEN_KEY."
+  #endif
+#endif
+
 #if EXTRUDERS > 1 && defined TEMP_SENSOR_1_AS_REDUNDANT
   #error "You cannot use TEMP_SENSOR_1_AS_REDUNDANT if EXTRUDERS > 1"
 #endif