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.

motion_control.h

@@ -44,8 +44,8 @@ void mc_arc(double *position, double *target, double *offset, uint8_t axis_0, ui
   uint8_t axis_linear, double feed_rate, uint8_t invert_feed_rate, double radius, uint8_t isclockwise);
 #endif
   
-// Dwell for a couple of time units
-void mc_dwell(uint32_t milliseconds);
+// Dwell for a specific number of seconds
+void mc_dwell(double seconds);
 
 // Send the tool home (not implemented)
 void mc_go_home();