Logging Connection Status and alarms for Blocks & PVs in NeXus files

[Kevin-Woods-Tessella]

As a scientist, I want IBEX to log disconnected blocks and PVs in the NeXus file, so that take account of missing or uncertain data when analysing my data files.

Acceptance Criteria

1. On encountering an INVALID alarm, IBEX will log blocks with their last-known-good-value and the alarm status.

Notes

1. The implementation details were discussed with Matt Jones (see e-mail chain titled Invalid values in NeXus files (dated: 06 June 2019 08:27).

[FreddieAkeroyd]

I have some ideas of how to do this, I think most of the stuff needed is actually already there

[JamesKingWork]

Is this actually in review? Is there a pull request for it?