G54 work coordinate support (w/ G10,G92.1). Re-factored g-code parser with error checking. Minor compiler compatibility changes.

- G54 work coordinate system support. Up to 6 work coordinate systems
(G54-G59) available as a compile-time option.

- G10 command added to set work coordinate offsets from machine
position.

- G92/G92.1 position offsets and cancellation support. Properly follows
NIST standard rules with other systems.

- G53 absolute override now works correctly with new coordinate systems.

- Revamped g-code parser with robust error checking. Providing user
feedback with bad commands. Follows NIST standards.

- Planner module slightly changed to only expected position movements
in terms of machine coordinates only. This was to simplify coordinate
system handling, which is done solely by the g-code parser.

- Upon grbl system abort, machine position and work positions are
retained, while G92 offsets are reset per NIST standards.

- Compiler compatibility update for _delay_us().

- Updated README.

main.c

@@ -53,15 +53,17 @@ int main(void)
     // reset to finish the initialization process.
     if (sys.abort) {
       
-      // Retain last known machine position. If the system abort occurred while in motion, machine
-      // position is not guaranteed, since a hard stop can cause the steppers to lose steps. Always
-      // perform a feedhold before an abort, if maintaining accurate machine position is required.
+      // Retain last known machine position and work coordinate offset(s). If the system abort
+      // occurred while in motion, machine position is not guaranteed, since a hard stop can cause
+      // the steppers to lose steps. Always perform a feedhold before an abort, if maintaining
+      // accurate machine position is required.
       // TODO: Report last position and coordinate offset to users to help relocate origins. Future
       // releases will auto-reset the machine position back to [0,0,0] if an abort is used while 
       // grbl is moving the machine.
-      int32_t last_position[3]; // last_coord_offset[3];
+      int32_t last_position[3];
+      double last_coord_system[N_COORDINATE_SYSTEM][3];
       memcpy(last_position, sys.position, sizeof(sys.position)); // last_position[] = sys.position[]
-      // memcpy(last_coord_offset, sys.coord_offset, sizeof(sys.coord_offset)); // last_coord_offset[] = sys.coord_offset[]
+      memcpy(last_coord_system, sys.coord_system, sizeof(sys.coord_system)); // last_coord_system[] = sys.coord_system[]
 
       // Reset system.
       memset(&sys, 0, sizeof(sys)); // Clear all system variables
@@ -74,8 +76,9 @@ int main(void)
       limits_init();
       st_reset(); // Clear stepper subsystem variables.
       
-      // Reload last known machine position. Coordinate offsets are reset per NIST RS274-NGC protocol.
+      // Reload last known machine position and work systems. G92 coordinate offsets are reset.
       memcpy(sys.position, last_position, sizeof(last_position)); // sys.position[] = last_position[]
+      memcpy(sys.coord_system, last_coord_system, sizeof(last_coord_system)); // sys.coord_system[] = last_coord_system[]
       gc_set_current_position(last_position[X_AXIS],last_position[Y_AXIS],last_position[Z_AXIS]);
       plan_set_current_position(last_position[X_AXIS],last_position[Y_AXIS],last_position[Z_AXIS]);
       
@@ -85,6 +88,7 @@ int main(void)
       #ifdef CYCLE_AUTO_START
         sys.auto_start = true;
       #endif
+      // TODO: Install G20/G21 unit default into settings and load appropriate settings.
     }
     
     protocol_execute_runtime();