Fixed minor bugs in planner. Increased max dwell time. Long slope bug stop-gap solution note.

- Fixed the planner TODO regarding minimum nominal speeds. Re-arranged
calculations to be both more efficient and guaranteed to be greater
than zero. - Missed a parenthesis location on the rate_delta
calculation. Should fix a nearly in-perceptible issue with incorrect
acceleration ramping in diagonal directions. - Increased maximum dwell
time from 6.5sec to an 18hour max. A crazy amount more, but that's how
the math works out. - Converted the internal feedrate values to mm/min
only, as it was switching between mm/min to mm/sec and back to mm/min.
Also added a feedrate > 0 check in gcode.c. - Identified the long slope
at the end of rapid de/ac-celerations noted by stephanix. Problem with
the numerical integration truncation error between the exact solution
of estimate_acceleration_distance and how grbl actually performs the
acceleration ramps discretely. Increasing the
ACCELERATION_TICKS_PER_SECOND in config.h helps fix this problem.
Investigating further.

config.h

@@ -55,6 +55,11 @@
 
 // The temporal resolution of the acceleration management subsystem. Higher number
 // give smoother acceleration but may impact performance
+// NOTE: Increasing this parameter will help remove the long slow motion bug at the end
+// of very fast de/ac-celerations. This is due to the increased resolution of the 
+// acceleration steps that more accurately predicted by the planner exact integration
+// of acceleration distance. An efficient solution to this bug is under investigation.
+// In general, setting this parameter is high as your system will allow is suggested.
 #define ACCELERATION_TICKS_PER_SECOND 40L
 
 #endif