DM-6965: multibandDriver: write metadata from detection

[PaulPrice]

We now put some useful information in the detection metadata, so we need
to make sure it's clean and write it out.

Not using MultiBandDriverTask.writeMetadata because the 'dataRef' that's
provided there is actually a list of data references, and it's not clear
we can identify which entry in the metadata belongs to which dataRef.

[TallJimbo]

I'm hoping that using detectCoaddSources.writeMetadata also means we get the same outputs when running DetectCoaddSourcesTask directly as a command-line Task, so I agree it's the right thing to do.

If you haven't already, could you check that doing that (running DetectCoaddSourcesTask directly) also writes metadata?

[PaulPrice]

Checked both ways of writing the metadata (detectCoaddSources.py and multibandDriver.py), and both get the value in there. When writing from multiband, you get an extra multiband-specific prefix to the key name (because the Task hierarchy contains that extra level, and the mechanics of writing the metadata respects that) but I figure that's not a big problem (if it is, we can address it on a separate ticket; at least the value is in there).