Added document logging & unit tests

[jackbdoughty]

• Creates a new callback function that the BlueSky run engine subscribes to which records every document in a log file under "C:\","instrument","var","logs","bluesky","raw_documents".
• Added a unit test that checks that the file to be written to is opened 3 times, for open, descriptor then stop. Also in the stop document to be written, checks that the run is successful with no interruptions.
• Callback returns an ID of 0 if it is ever needed to unsubscribe

Ticket6

[Tom-Willemsen]

Could you write some docs (just some markdown in the docs folder) about what this does, how it works, where the logs go, how to use it for debugging etc?

[Tom-Willemsen]

Looks good, thanks.

We might need to do something cleverer if we ever end up doing nested multi-run plans, but this is fine for the simple plans we'll be doing in the immediate future.