genie_python: Extend waitfor to allow use to see what is being waited for

As an instrument scientist it would be nice to know why my instrument is waiting during a `waitfor_blah` operation. This information should be displayed on the dataweb. Extend genie_python `waitfor...` to write the reason for the wait to a PV which can be accessed by the dataweb.

Idea from sprint backlog prep.

Start by locating the genie_python waitfor... entry points and tracing how the dataweb accesses process variables. Identify the PV used for the wait reason and the affected waitfor_blah paths, then verify that a reason is written during a wait and displayed by the dataweb.

Scritto dal modello di indicizzazione a partire dal testo della issue.