DPT 251.600 RGBW added

[korpa]

This pull request adds DPT 251.600. As it is a combined data type I thought the best solution is to create a struct for the 4 color unit8 and 4 color valid bits.

What do you think?

	orange := dpt.DPT_251600{Red: 255, Green: 96, Blue: 0, White: 18, RedValid: true, GreenValid: true, BlueValid: true, WhiteValid: true}

	err = client.Send(knx.GroupEvent{
		Command:     knx.GroupWrite,
		Destination: cemi.NewGroupAddr3(3, 0, 0),
		Data:        orange.Pack(),
	})
	if err != nil {
		log.Fatal(err)
	}

[vapourismo]

Thank you very much for the contribution!

There are just a few things to be addressed, then it can be merged :)

[vapourismo]

This should be of type byte (or uint8).

[korpa]

Thank you very much. Then I can remove following line of my go.mod :)

replace github.com/vapourismo/knx-go => ../../knx-go