Unit specific concepts introduced

example/example.cpp

@@ -25,11 +25,11 @@
 
 using namespace units;
 
-template<Unit U1, typename Rep1, Unit U2, typename Rep2>
-void foo(velocity<U1, Rep1> v, time<U2, Rep2> t)
+template<typename V, typename T>
+  requires Velocity<V> && Time<T>
+void foo(V v, T t)
 {
   const auto distance = v * t;
-
   std::cout << "A car driving " << v.count() << " km/h in a time of " << t.count() << " minutes will pass "
             << quantity_cast<length<meter, int>>(distance).count() << " meters.\n";
 }