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.

config.h

@@ -65,6 +65,10 @@
 #define CMD_CYCLE_START '~'
 #define CMD_RESET 0x18 // ctrl-x
 
+// Specifies the number of work coordinate systems grbl will support (G54 - G59).
+// This parameter must be one or greater, currently supporting up to a value of 6.
+#define N_COORDINATE_SYSTEM 1
+
 // This parameter sets the delay time before disabling the steppers after the final block of movement.
 // A short delay ensures the steppers come to a complete stop and the residual inertial force in the 
 // CNC axes don't cause the axes to drift off position. This is particularly important when manually