Muons: set run "comment" from bluesky scan

As a user on muons I want to be able to find what scan a DAE .nxs file was part of.

To do this, insert the bluesky scan ID (or another suitable string identifier) as the "run comment" in the experiment details. This way we have a two-way link between .nxs which has a pointer to the scan it's part of, and the scan which has pointers to the .nxs it generated.

We can write this as a bluesky preprocessor that reacts to open_run and close_run messages. After we've seen the open_run message we know the UID of a scan, the preprocessor can then write this to the comment field. On close run it should put the comment back to whatever it was before.