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.c

@@ -29,13 +29,21 @@
 #include "stepper.h"
 #include "planner.h"
 
-#define N_ARC_CORRECTION 25    // (0-255) Number of iterations before arc trajectory correction
+// Number of arc generation iterations with small angle approximation before exact arc
+// trajectory correction. Value must be 1-255.
+#define N_ARC_CORRECTION 25 
 
 
-void mc_dwell(uint32_t milliseconds) 
+// Execute dwell in seconds. Maximum time delay is > 18 hours, more than enough for any application.
+void mc_dwell(double seconds) 
 {
-  st_synchronize();
-  _delay_ms(milliseconds);
+   uint16_t i = floor(seconds);
+   st_synchronize();
+   _delay_ms(floor(1000*(seconds-i))); // Delay millisecond remainder
+   while (i > 0) {
+     _delay_ms(1000); // Delay one second
+     i--;
+   }
 }
 
 // Execute an arc in offset mode format. position == current xyz, target == target xyz,