Add negation for DiscreteAngle

include/DiscreteAngle.h

@@ -7,6 +7,8 @@
 class DiscreteAngle16 : public Range16 {
 public:
   DiscreteAngle16(float angle);
+
+  inline DiscreteAngle16 &operator-();
 };
 
 #endif

src/DiscreteAngle.cpp

@@ -22,3 +22,5 @@ DiscreteAngle16::DiscreteAngle16(float angle) : Range16() {
   // This means that range value 0 is not used
   this->range = (unsigned short)((angle + 180.0F) * 65535.0F);
 }
+
+DiscreteAngle16 &DiscreteAngle16::operator-() { -this->range; }