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.

planner.h

@@ -69,9 +69,6 @@ block_t *plan_get_current_block();
 // Reset the planner position vector (in steps)
 void plan_set_current_position(int32_t x, int32_t y, int32_t z);
 
-// Apply G92 coordinate offsets and update planner position vector. Called by g-code parser.
-void plan_set_coordinate_offset(double x, double y, double z);
-
 // Reinitialize plan with a partially completed block
 void plan_cycle_reinitialize(int32_t step_events_remaining);