G38.2 probe feature rough draft installed. Working but needs testing.

- G38.2 straight probe now supported. Rough draft. May be tweaked more
as testing ramps up.

- G38.2 requires at least one axis word. Multiple axis words work too.
When commanded, the probe cycle will move at the last ‘F’ feed rate
specified in a straight line.

- During a probe cycle: If the probe pin goes low (normal high), Grbl
will record that immediate position and engage a feed hold. Meaning
that the CNC machine will move a little past the probe switch point, so
keep federates low to stop sooner. Once stopped, Grbl will issue a move
to go back to the recorded probe trigger point.

- During a probe cycle: If the probe switch does not engage by the time
the machine has traveled to its target coordinates, Grbl will issue an
ALARM and the user will be forced to reset Grbl. (Currently G38.3 probe
without error isn’t supported, but would be easy to implement later.)

- After a successful probe, Grbl will send a feedback message
containing the recorded probe coordinates in the machine coordinate
system. This is as the g-code standard on probe parameters specifies.

- The recorded probe parameters are retained in Grbl memory and can be
viewed with the ‘$#’ print parameters command. Upon a power-cycle, not
a soft-reset, Grbl will re-zero these values.

- Moved ‘$#’ command to require IDLE or ALARM mode, because it accesses
EEPROM to fetch the coordinate system offsets.

- Updated the Grbl version to v0.9d.

- The probe cycle is subject to change upon testing or user-feedback.

gcode.h

@@ -31,7 +31,7 @@
 // and are similar/identical to other g-code interpreters by manufacturers (Haas,Fanuc,Mazak,etc).
 #define MODAL_GROUP_NONE 0
 #define MODAL_GROUP_0 1 // [G4,G10,G28,G30,G53,G92,G92.1] Non-modal
-#define MODAL_GROUP_1 2 // [G0,G1,G2,G3,G80] Motion
+#define MODAL_GROUP_1 2 // [G0,G1,G2,G3,G38.2,G80] Motion
 #define MODAL_GROUP_2 3 // [G17,G18,G19] Plane selection
 #define MODAL_GROUP_3 4 // [G90,G91] Distance mode
 #define MODAL_GROUP_4 5 // [M0,M1,M2,M30] Stopping
@@ -47,7 +47,8 @@
 #define MOTION_MODE_LINEAR 1 // G1
 #define MOTION_MODE_CW_ARC 2  // G2
 #define MOTION_MODE_CCW_ARC 3  // G3
-#define MOTION_MODE_CANCEL 4 // G80
+#define MOTION_MODE_PROBE 4 // G38.x
+#define MOTION_MODE_CANCEL 5 // G80
 
 #define PROGRAM_FLOW_RUNNING 0
 #define PROGRAM_FLOW_PAUSED 1 // M0, M1
@@ -60,10 +61,8 @@
 #define NON_MODAL_SET_HOME_0 4 // G28.1
 #define NON_MODAL_GO_HOME_1 5 // G30
 #define NON_MODAL_SET_HOME_1 6 // G30.1
-#define NON_MODAL_PROBE_WITH_ERROR 7 //G38.2
-#define NON_MODAL_PROBE_NO_ERROR 8 //G38.3 
-#define NON_MODAL_SET_COORDINATE_OFFSET 9 // G92
-#define NON_MODAL_RESET_COORDINATE_OFFSET 10 //G92.1
+#define NON_MODAL_SET_COORDINATE_OFFSET 7 // G92
+#define NON_MODAL_RESET_COORDINATE_OFFSET 8 //G92.1
 
 typedef struct {
   uint8_t status_code;             // Parser status for current block