interp: fix GE/LE operators to better handle double-precision comparison

This commit does four things:
1. Changes the LE/GE operators to better handle double-precision 
floating-point comparisons. When comparing two values as equal (or not 
equal) the EQ and NE operators considered the floating-point precision 
"issue" by comparing the difference between the two values to 
TOLERANCE_EQUAL, GE and LE did not.
2. Tidy the difference by using fabs()
3. Modify the TOLERANCE_EQUAL threshold to 1e-6.  Previously this was 
set to .0001 which is very close to a reasonable difference between two 
gcode coordinate values.
4. Update the documentation to reflect these change.

docs/src/gcode/overview.adoc

@@ -704,11 +704,10 @@ false, and any non-zero number is equivalent to logical true.
 
 == Equality and floating-point values
 
-The RS274/NGC language only supports floating-point values of finite
-precision. Therefore, testing for equality or inequality of two
-floating-point values is inherently problematic. The interpreter
-solves this problem by considering values equal if their absolute
-difference is less than 0.0001 (this value is defined as
+Testing for equality or inequality of two
+double-precision floating-point values is inherently problematic.
+The interpreter solves this problem by considering values equal if
+their absolute difference is less than 1e-6 (this value is defined as
 'TOLERANCE_EQUAL' in src/emc/rs274ngc/interp_internal.hh).
 
 [[gcode:functions]]

src/emc/rs274ngc/interp_execute.cc

@@ -158,20 +158,20 @@ int Interp::execute_binary2(double *left,        //!< pointer to the left operan
       *left = (*left < *right) ? 1.0 : 0.0;
       break;
   case EQ:
-      diff = *left - *right;
-      diff = (diff < 0) ? -diff : diff;
+      diff = fabs(*left - *right);
       *left = (diff < TOLERANCE_EQUAL) ? 1.0 : 0.0;
       break;
   case NE:
-      diff = *left - *right;
-      diff = (diff < 0) ? -diff : diff;
+      diff = fabs(*left - *right);
       *left = (diff >= TOLERANCE_EQUAL) ? 1.0 : 0.0;
       break;
   case LE:
-      *left = (*left <= *right) ? 1.0 : 0.0;
+      diff = fabs(*left - *right);
+      *left = ((diff < TOLERANCE_EQUAL) || (*left <= *right)) ? 1.0 : 0.0;
       break;
   case GE:
-      *left = (*left >= *right) ? 1.0 : 0.0;
+      diff = fabs(*left - *right);
+      *left = ((diff < TOLERANCE_EQUAL) || (*left >= *right)) ? 1.0 : 0.0;
       break;
   case GT:
       *left = (*left > *right) ? 1.0 : 0.0;

src/emc/rs274ngc/interp_internal.hh

@@ -87,8 +87,8 @@ Tighter tolerance down to a minimum of 1 micron +- also accepted.
 #define SPIRAL_RELATIVE_TOLERANCE 0.001
 
 /* angle threshold for concavity for cutter compensation, in radians */
-#define TOLERANCE_CONCAVE_CORNER 0.05  
-#define TOLERANCE_EQUAL 0.0001 /* two numbers compare EQ if the
+#define TOLERANCE_CONCAVE_CORNER 0.05
+#define TOLERANCE_EQUAL 1e-6 /* two numbers compare EQ if the
 				  difference is less than this */
 
 static inline bool equal(double a, double b)