Significantly improved junction control and fixed computation bugs in planner

- Junction jerk now re-defined as junction_deviation. The distance from
the junction to the edge of a circle tangent to both previous and
current path lines. The circle radii is used to compute the maximum
junction velocity by centripetal acceleration. More robust and
simplified way to compute jerk.   - Fixed bugs related to entry and exit
factors. They were computed based on the current nominal speeds but not
when computing exit factors for neighboring blocks. Removed factors and
replaced with entry speeds only. Factors now only computed for stepper
trapezoid rate conversions.  - Misc: Added min(), next_block_index,
prev_block_index functions for clarity.

settings.h

@@ -41,7 +41,7 @@ typedef struct {
   uint8_t invert_mask;
   double mm_per_arc_segment;
   double acceleration;
-  double max_jerk;
+  double junction_deviation;
 } settings_t;
 extern settings_t settings;