add Chapter 15 - runtime observer implementation

Chapter15/observer/util/src/units.cpp

@@ -0,0 +1,11 @@
+#include <units.hpp>
+
+namespace units {
+voltage operator""_V(long double volts) {
+    return voltage(static_cast<float>(volts));
+}
+
+resistance operator""_Ohm(long double ohms) {
+    return resistance(static_cast<float>(ohms));
+}
+};