probe: Add ability to multi-sample points to ProbePointsHelper

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

config/example-extras.cfg

@@ -69,6 +69,12 @@
 #   NEXT extended g-code command to record the position at that
 #   point. The default is false if a [probe] config section is present
 #   and true otherwise.
+#samples: 1
+#   The number of times to probe each point.  The probed z-values
+#   will be averaged.  The default is to probe 1 time.
+#sample_retract_dist: 2.0
+#   The distance (in mm) to retract between each sample if
+#   sampling more than once.  Default is 2mm.
 
 
 # Mesh Bed Leveling. One may define a [bed_mesh] config section
@@ -82,6 +88,12 @@
 #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
+#   The number of times to probe each point.  The probed z-values
+#   will be averaged.  The default is to probe 1 time.
+#sample_retract_dist: 2.0
+#   The distance (in mm) to retract between each sample if
+#   sampling more than once.  Default is 2mm.
 #min_point:
 #   An X,Y point defining the minimum coordinate to probe on
 #   the bed. Note that this refers to the nozzle position,
@@ -150,7 +162,12 @@
 #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
+#   The number of times to probe each point.  The probed z-values
+#   will be averaged.  The default is to probe 1 time.
+#sample_retract_dist: 2.0
+#   The distance (in mm) to retract between each sample if
+#   sampling more than once.  Default is 2mm.
 
 # In a multi-extruder printer add an additional extruder section for
 # each additional extruder. The additional extruder sections should be