quad_gantry_level: Add gantry leveling (ruled hyperbolic parabola correction) (#631)

Signed-off-by: Maks Zolin <mzolin@vorondesign.com>

config/example-extras.cfg

@@ -933,3 +933,43 @@
 # Replicape support - see the generic-replicape.cfg file for further
 # details.
 #[replicape]
+
+# Moving gantry leveling using 4 independently controlled Z motors.
+# Corrects hyperbolic parabola effects (potato chip) on moving gantry
+# which is more flexible.
+# WARNING: Using this on a moving bed may lead to undesirable results.
+# If this section is present then a QUAD_GANTRY_LEVEL extended G-Code
+# command becomes available. This routine assumes the following Z motor
+# configuration:
+# ----------------
+# |Z1          Z2|
+# |  ---------   |
+# |  |       |   |
+# |  |       |   |
+# |  x--------   |
+# |Z           Z3|
+# ----------------
+# Where x is the (0,0) point on the bed
+#[quad_gantry_level]
+#gantry_corners:
+#   A newline separated list of X,Y coordinates describing the
+#   two opposing corners of the gantry. The first entry corresponds to
+#   Z, the second to Z2.
+#   This parameter must be provided.
+#points:
+#   A newline separated list of four X,Y points that should be probed
+#   during a QUAD_GANTRY_LEVEL command.
+#   Order of the locations is important, and should correspond to Z,Z1
+#   Z2, and Z3 location in order.
+#   This parameter must be provided.
+#   For maximum accuracy, ensure your probe offsets are configured.
+#speed: 50
+#   The speed (in mm/s) of non-probing moves during the calibration.
+#   The default is 50.
+#horizontal_move_z: 5
+#   The height (in mm) that the head should be commanded to move to
+#   just prior to starting a probe operation. The default is 5
+#samples: 1
+#   Number of probe samples per point. The defaut is 1
+#sample_retract_dist: 2.0
+#   Distance in mm to retract the probe between samples. Default is 2.