Add medication dosage example

examples/medication_dosage.nbt

@@ -0,0 +1,14 @@
+@aliases(takings)
+unit taking
+
+let body_weight = 75 kg
+
+let dosage = (60 mg / kg) / day
+
+let frequency = 3 takings / day
+
+let total_daily_dose = dosage * body_weight -> mg / day
+assert_eq(total_daily_dose, 4500 mg/day)
+
+let dose = total_daily_dose / frequency -> mg / taking
+assert_eq(dose, 1500 mg/taking)