arduino: Output pins that denote relays

It is proposed to introduce pin notation to denote an output pin corresponding to a relay, e.g., K1 for relay 1. The GPIO pin corresponding to the relay need not be known to the user, since it would be compiled into the NetSender client.

Furthermore, power variables, such as Power1, would be automatically associated with the corresponding relay, K1, without the need for an explicit actuator, thereby reducing the amount of configuration required.