Add CargoTypesBit helper method to CargoSpec

src/cargotype.h

@@ -90,6 +90,15 @@ struct CargoSpec {
 		return this - CargoSpec::array;
 	}
 
+	/**
+	 * Determine CargoTypes bit of this cargospec
+	 * @return CargoTypes bit
+	 */
+	inline CargoTypes CargoTypesBit() const
+	{
+		return static_cast<CargoTypes>(1) << this->Index();
+	}
+
 	/**
 	 * Tests for validity of this cargospec
 	 * @return is this cargospec valid?