Issue in Arc::GetHeadingAt for arc geometry

Issue description:

It appears that there may be a bug in the Arc::GetHeadingAt function in arc.cpp.

e.g.

<geometry hdg="4.7998015677923114" length="54.387626262626242" s="265.68821028424395" x="1085.6618554277859" y="1237.2872167871139">
  <arc curvature="0.0076923076923076927" />
</geometry>

when get road orientation at s = 266_m, the expected value would be some value near 4.7998015677923114, but got 6.8459554.

The conversion needs to be checked.