Move Centroid to coordinate_calculation

src/util/coordinate_calculation.cpp

@@ -200,6 +200,16 @@ double perpendicularDistanceFromProjectedCoordinate(
     return approximate_distance;
 }
 
+Coordinate centroid(const Coordinate lhs, const Coordinate rhs)
+{
+    Coordinate centroid;
+    // The coordinates of the midpoints are given by:
+    // x = (x1 + x2) /2 and y = (y1 + y2) /2.
+    centroid.lon = (lhs.lon + rhs.lon) / FixedLongitude(2);
+    centroid.lat = (lhs.lat + rhs.lat) / FixedLatitude(2);
+    return centroid;
+}
+
 double degToRad(const double degree)
 {
     using namespace boost::math::constants;