ADD: new track message, Entity class and Position class

libs/geographiclib/examples/example-GravityModel.cpp

@@ -0,0 +1,24 @@
+// Example of using the GeographicLib::GravityModel class
+// This requires that the egm96 gravity model be installed; see
+// https://geographiclib.sourceforge.io/C++/doc/gravity.html#gravityinst
+
+#include <iostream>
+#include <exception>
+#include <GeographicLib/GravityModel.hpp>
+
+using namespace std;
+using namespace GeographicLib;
+
+int main() {
+  try {
+    GravityModel grav("egm96");
+    double lat = 27.99, lon = 86.93, h = 8820; // Mt Everest
+    double gx, gy, gz;
+    grav.Gravity(lat,lon, h, gx, gy, gz);
+    cout << gx << " " << gy << " " << gz << "\n";
+  }
+  catch (const exception& e) {
+    cerr << "Caught exception: " << e.what() << "\n";
+    return 1;
+  }
+}