Validate fixture addressing

[craftyjon]

Life is a lot easier if we can validate the addresses. Most importantly, fixtures on a strand need to have continuous addressing from 0 to N. Strands themselves don't necessarily need to have continuous addressing, since each strand is handled by its own controller. We can save on network overhead if a bulk strand write can leave out the fixture addresses by assuming that they cover a continuous range.

[zbenjamin]

We now validate that fixtures have unique addresses, but not that they're consecutive. The default is to create consecutive addresses, though, so it's easier for the user to get it right.

[craftyjon]

TBD: This issue might be moot with the move to PixelGroups and offset-based addressing