Add missing setRGB and getRGB functions

[mbarbey]

In the documentation, it's explained that the getRGB and setRGB functions are available on both Light ang Group objects.

Unfortunately it appear that these functions are (currently) only available on the Light object.

So here is my little copy/past to add these functions to the Group object.

[mbarbey]

provided :

• get/set RGB function on the Group object
• unit test for get/set RGB function on the Group object

[sqmk]

Merged and tagged as v1.6.1. Thank you!