quad_gantry_level: Support retries

Support retrying QUAD_GANTRY_LEVEL a configurable number of times
to a configurable tolerance both in the config or as parameters.

- By default keeps original behavior of no retries.
- Adds parameters RETRIES and RETRY_TOLERANCE to QUAD_GANTRY_LEVEL gcode.
- adds config options retries and retry_tolerance to `[quad_gantry_level]`
- issues an error if we are getting worse intead of approaching tolerance
- issues an error if retries were requested but we did not reach the tolerance
in the specified number of retries

Signed-off-by: John "Fess" Fessenden <fess@fess.org>

config/example-extras.cfg

@@ -350,6 +350,11 @@
 #max_adjust: 4
 #   Saftey limit if an ajustment greater than this value is requested
 #   quad_gantry_level will abort.
+#retries: 0
+#   number of times to retry if the probed points aren't within tolerance
+#retry_tolerance: 0
+#   if retries are enabled then retry if largest and smallest probed points
+#   differ more than retry_tolerance
 
 
 # In a multi-extruder printer add an additional extruder section for