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.

config.h

@@ -42,6 +42,7 @@
 
 #define LIMIT_DDR       DDRB
 #define LIMIT_PIN       PINB
+#define LIMIT_PORT      PORTB
 #define X_LIMIT_BIT     1  // Uno Digital Pin 9
 #define Y_LIMIT_BIT     2  // Uno Digital Pin 10
 #define Z_LIMIT_BIT     3  // Uno Digital Pin 11
@@ -175,7 +176,5 @@
 
 //  Limit step rate for homing
 #define LIMIT_DEBOUNCE 50  // Limit switch debounce delay (in ms)
-// #define LIMIT_INVERT_MASK 0 //
-// #define LIMIT_NORMAL_HIGH 1 // Normal low 0 or normal high 1
 
 #endif