Support running an "LSM" without a detector

[marktsuchida]

This is one of the requirements for controlling a scanner for photomanipulation (as opposed to image acquisition).

Rather than distinguishing between full LSMs and scan-only configurations, we might just allow LSMs to have zero channels.

A typical "dual-scanner" system would have one LSM with scanner and detector (for acquisition) and another LSM with a real scanner and a "dummy" detector producing zero channels (for photomanipulation).