Limit pin internal pull-resistors enabled. Re-wrote read_double() function. Correctly changed all 'double's to 'float's.

- Limit pin internal pull-resistors now enabled. Normal high operation.
This will be the standard going forward.

- Updated all of the 'double' variable types to 'float' to reflect what
happens when compiled for the Arduino. Also done for compatibility
reasons to @jgeisler0303 's Grbl simulator code.

- G-code parser will now ignore 'E' exponent values, since they are
reserved g-code characters for some machines. Thanks @csdexter!

- The read_double() function was re-written and optimized for use in
Grbl. The strtod() avr lib was removed.

motion_control.h

@@ -28,17 +28,17 @@
 // Execute linear motion in absolute millimeter coordinates. Feed rate given in millimeters/second
 // unless invert_feed_rate is true. Then the feed_rate means that the motion should be completed in
 // (1 minute)/feed_rate time.
-void mc_line(double x, double y, double z, double feed_rate, uint8_t invert_feed_rate);
+void mc_line(float x, float y, float z, float feed_rate, uint8_t invert_feed_rate);
 
 // Execute an arc in offset mode format. position == current xyz, target == target xyz, 
 // offset == offset from current xyz, axis_XXX defines circle plane in tool space, axis_linear is
 // the direction of helical travel, radius == circle radius, isclockwise boolean. Used
 // for vector transformation direction.
-void mc_arc(double *position, double *target, double *offset, uint8_t axis_0, uint8_t axis_1,
-  uint8_t axis_linear, double feed_rate, uint8_t invert_feed_rate, double radius, uint8_t isclockwise);
+void mc_arc(float *position, float *target, float *offset, uint8_t axis_0, uint8_t axis_1,
+  uint8_t axis_linear, float feed_rate, uint8_t invert_feed_rate, float radius, uint8_t isclockwise);
   
 // Dwell for a specific number of seconds
-void mc_dwell(double seconds);
+void mc_dwell(float seconds);
 
 // Send the tool home (not implemented)
 void mc_go_home();