[example] Add example on distance formula

The formula used is Karney's direct method.

example/ml03_distance_formula.cpp

@@ -0,0 +1,37 @@
+// Boost.Geometry
+
+// Copyright (c) 2018 Adeel Ahmad, Islamabad, Pakistan.
+
+// Contributed and/or modified by Adeel Ahmad, as part of Google Summer of Code 2018 program.
+
+// Use, modification and distribution is subject to the Boost Software License,
+// Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+// Formula example - Show how to use Karney's direct method.
+
+#include <boost/geometry.hpp>
+#include <boost/geometry/formulas/karney_direct.hpp>
+
+using namespace boost::geometry;
+
+int main()
+{
+    double lon1_deg = 0.;
+    double lat1_deg = 73.114273316483;
+    double distance_m = 19992866.6147806;
+    double azi12_deg = 78.154765899661;
+
+    // Create an alias of the formula.
+    typedef formula::karney_direct<double, true, true, true, true, 8> karney_direct;
+
+    // Structure to hold the resulting values.
+    formula::result_direct<double> result;
+
+    // WGS-84 spheroid.
+    srs::spheroid<double> spheroid(6378137.0, 6356752.3142451793);
+
+    result = karney_direct::apply(lon1_deg, lat1_deg, distance_m, azi12_deg, spheroid);
+
+    return 0;
+}