Add 'xo-unit/' from commit 'b531e382c2'

git-subtree-dir: xo-unit
git-subtree-mainline: e9ee6992ca
git-subtree-split: b531e382c2

xo-unit/example/ex3/ex3.cpp

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+/** @file ex3.cpp **/
+
+#include "xo/unit/quantity.hpp"
+#include "xo/unit/quantity_iostream.hpp"
+#include <iostream>
+
+int
+main () {
+    namespace q = xo::qty::qty;
+    namespace u = xo::qty::u;
+    using xo::qty::quantity;
+    using namespace std;
+
+    constexpr quantity<u::second> t = q::minutes(2);
+    constexpr quantity<u::meter> d = q::kilometers(2.5);
+
+    constexpr auto t2 = t*t;
+    constexpr auto a = d / (t*t);
+
+    cerr << "t: " << t << ", d: " << d
+         << ", d.t^-2: " << a
+         << endl;
+}
+
+/** end ex3.cpp **/