added new sanity-check assertions

fixed issue with angles [bad interface]
- adjusted other parts accordingly
- added corresponding test-cases
started working on absolute heading

geo/Heading.h

@@ -33,8 +33,11 @@ public:
 	}
 
 	/** signled angular difference [-PI:+PI] */
-	float getSignedDiff(const Heading other) const {
-		return Angle::getSignedDiffRAD_2PI(rad, other.rad);
+//	float getSignedDiff(const Heading other) const {
+//		return Angle::getSignedDiffRAD_2PI(other.rad, rad);
+//	}
+	static float getSignedDiff(const Heading from, const Heading to) {
+		return Angle::getSignedDiffRAD_2PI(from.rad, to.rad);
 	}
 
 	/** update the angle but ensure we stay within [0:2PI] */