Minor bug fixes and updates. Line number tracking.

- Line number tracking was getting truncated at 255, since it was using
wrong variable type. Fixed it with a trunc().

- Increased the max number line allowed by Grbl to 9999999 from the
g-code standard 99999. The latter seems to be an arbitrary number, so
we are allowing larger ones for at least one known use case scenario.

- Created a new test directory to contain some testing g-code to proof
the firmware. Only got started with one test case so far. More will be
inserted as needed.

- Some other commenting updates to clarify certain aspects of the code.

nuts_bolts.c

@@ -151,6 +151,7 @@ uint8_t get_direction_mask(uint8_t axis_idx)
   return(axis_mask);
 }
 
+
 float hypot_f(float x, float y)
 {
    return(sqrt(x*x + y*y));