Minor update for memory savings in ring buffer and fleshed out commenting.

No changes in functionality. Path vectors moved from ring buffer to
local planner static variables to save 3*(BUFFER_SIZE - 1) doubles in
memory. Detailed comments. Really need to stop micro-updating. Should be
the last until a planner optimization (ala Jens Geisler) has been
completed.

nuts_bolts.h

@@ -32,6 +32,7 @@
 #define Z_AXIS 2
 
 #define clear_vector(a) memset(a, 0, sizeof(a))
+#define clear_vector_double(a) memset(a, 0.0, sizeof(a))
 #define max(a,b) (((a) > (b)) ? (a) : (b))
 #define min(a,b) (((a) < (b)) ? (a) : (b))