ADD: new track message, Entity class and Position class

libs/geographiclib/examples/example-Accumulator.cpp

@@ -0,0 +1,24 @@
+// Example of using the GeographicLib::Accumulator class
+
+#include <iostream>
+#include <exception>
+#include <GeographicLib/Accumulator.hpp>
+
+using namespace std;
+using namespace GeographicLib;
+
+int main() {
+  try {
+    // Compare using Accumulator and ordinary summation for a sum of large and
+    // small terms.
+    double sum = 0;
+    Accumulator<> acc = 0;
+    sum += 1e20; sum += 1; sum += 2; sum += 100; sum += 5000; sum += -1e20;
+    acc += 1e20; acc += 1; acc += 2; acc += 100; acc += 5000; acc += -1e20;
+    cout << sum << " " << acc() << "\n";
+  }
+  catch (const exception& e) {
+    cerr << "Caught exception: " << e.what() << "\n";
+    return 1;
+  }
+}