Max velocity axes independence installed. Fixed intermittent slow trailing steps. Timer0 disable fix.

- Maximum velocity for each axis is now configurable in settings. All
rapids/seek move at these maximums. All feed rates(including rapids)
may be limited and scaled down so that no axis does not exceed their
limits.

- Moved around auto-cycle start. May change later, but mainly to ensure
the planner buffer is completely full before cycle starting a streaming
program. Otherwise it should auto-start when there is a break in the
serial stream.

- Reverted old block->max_entry_speed_sqr calculations. Feedrate
overrides not close to ready at all.

- Fixed intermittent slow trailing steps for some triangle velocity
profile moves. The acceleration tick counter updating was corrected to
be exact for that particular transition. Should be ok for normal
trapezoidal profiles.

- Fixed the Timer0 disable after a step pulse falling edge. Thanks
@blinkenlight!

gcode.c

@@ -51,8 +51,7 @@ static void select_plane(uint8_t axis_0, uint8_t axis_1, uint8_t axis_2)
 void gc_init() 
 {
   memset(&gc, 0, sizeof(gc));
-  gc.feed_rate = settings.default_feed_rate; // Should be zero at initialization.
-//  gc.seek_rate = settings.default_seek_rate;
+  gc.feed_rate = settings.default_feed_rate;
   select_plane(X_AXIS, Y_AXIS, Z_AXIS);
   gc.absolute_mode = true;
   
@@ -247,9 +246,7 @@ uint8_t gc_execute_line(char *line)
      NOTE: Independent non-motion/settings parameters are set out of this order for code efficiency 
      and simplicity purposes, but this should not affect proper g-code execution. */
   
-  // ([F]: Set feed and seek rates.)
-  // TODO: Seek rates can change depending on the direction and maximum speeds of each axes. When
-  // max axis speed is installed, the calculation can be performed here, or maybe in the planner.
+  // ([F]: Set feed rate.)
     
   if (sys.state != STATE_CHECK_MODE) { 
     //  ([M6]: Tool change should be executed here.)
@@ -324,14 +321,14 @@ uint8_t gc_execute_line(char *line)
             target[i] = gc.position[i];
           }
         }
-        mc_line(target[X_AXIS], target[Y_AXIS], target[Z_AXIS], settings.default_seek_rate, false);
+        mc_line(target[X_AXIS], target[Y_AXIS], target[Z_AXIS], -1.0, false);
       }
       // Retreive G28/30 go-home position data (in machine coordinates) from EEPROM
       float coord_data[N_AXIS];
       uint8_t home_select = SETTING_INDEX_G28;
       if (non_modal_action == NON_MODAL_GO_HOME_1) { home_select = SETTING_INDEX_G30; }
       if (!settings_read_coord_data(home_select,coord_data)) { return(STATUS_SETTING_READ_FAIL); }
-      mc_line(coord_data[X_AXIS], coord_data[Y_AXIS], coord_data[Z_AXIS], settings.default_seek_rate, false); 
+      mc_line(coord_data[X_AXIS], coord_data[Y_AXIS], coord_data[Z_AXIS], -1.0, false); 
       memcpy(gc.position, coord_data, sizeof(coord_data)); // gc.position[] = coord_data[];
       axis_words = 0; // Axis words used. Lock out from motion modes by clearing flags.
       break;
@@ -347,7 +344,7 @@ uint8_t gc_execute_line(char *line)
         // Update axes defined only in block. Offsets current system to defined value. Does not update when
         // active coordinate system is selected, but is still active unless G92.1 disables it. 
         uint8_t i;
-        for (i=0; i<=2; i++) { // Axes indices are consistent, so loop may be used.
+        for (i=0; i<N_AXIS; i++) { // Axes indices are consistent, so loop may be used.
           if (bit_istrue(axis_words,bit(i)) ) {
             gc.coord_offset[i] = gc.position[i]-gc.coord_system[i]-target[i];
           }
@@ -382,7 +379,7 @@ uint8_t gc_execute_line(char *line)
     // absolute mode coordinate offsets or incremental mode offsets.
     // NOTE: Tool offsets may be appended to these conversions when/if this feature is added.
     uint8_t i;
-    for (i=0; i<=2; i++) { // Axes indices are consistent, so loop may be used to save flash space.
+    for (i=0; i<N_AXIS; i++) { // Axes indices are consistent, so loop may be used to save flash space.
       if ( bit_istrue(axis_words,bit(i)) ) {
         if (!absolute_override) { // Do not update target in absolute override mode
           if (gc.absolute_mode) {
@@ -402,7 +399,7 @@ uint8_t gc_execute_line(char *line)
         break;
       case MOTION_MODE_SEEK:
         if (!axis_words) { FAIL(STATUS_INVALID_STATEMENT);} 
-        else { mc_line(target[X_AXIS], target[Y_AXIS], target[Z_AXIS], settings.default_seek_rate, false); }
+        else { mc_line(target[X_AXIS], target[Y_AXIS], target[Z_AXIS], -1.0, false); }
         break;
       case MOTION_MODE_LINEAR:
         // TODO: Inverse time requires F-word with each statement. Need to do a check. Also need