Rare planner bug fix and added simulator defaults.

- Planner bug when moving along a diagonal back and forth on the same
path. Rare for the fact that most CAM programs don’t program this type
of motion, neither does jogging. Fixed in this update.

- Added grbl_sim defaults for testing purposes.

system.c

@@ -214,6 +214,9 @@ uint8_t system_execute_line(char *line)
 }
 
 
+// Returns machine position of axis 'idx'. Must be sent a 'step' array.
+// NOTE: If motor steps and machine position are not in the same coordinate frame, this function
+//   serves as a central place to compute the transformation.
 float system_convert_axis_steps_to_mpos(int32_t *steps, uint8_t idx)
 {
   float pos;