ADD: new track message, Entity class and Position class

libs/geographiclib/examples/example-Constants.cpp

@@ -0,0 +1,20 @@
+// Example of using the GeographicLib::Constants class
+
+#include <iostream>
+#include <exception>
+#include <GeographicLib/Constants.hpp>
+
+using namespace std;
+using namespace GeographicLib;
+
+int main() {
+  try {
+    cout << "WGS84 parameters:\n"
+         << "a = " << Constants::WGS84_a() << " m\n"
+         << "f = 1/" << 1/Constants::WGS84_f() << "\n";
+  }
+  catch (const exception& e) {
+    cerr << "Caught exception: " << e.what() << "\n";
+    return 1;
+  }
+}