`filewriter`: `isis_vms_compat`

Mantid currently loads some information from the `isis_vms_compat` group.

Primarily this is `UDET` and `SPEC`, information which doesn't appear elsewhere in the `.nxs` file, but if the `isis_vms_compat` group exists at all then mantid apparently needs all of:

> ['IRPB', 'NSP1', 'RRPB', 'RSPB', 'SPB', 'SPEC', 'UDET']

Documentation for what some of the old VMS names mean is available at:

```
\\isis\shares\ISIS_Experiment_Controls\data for tickets\ds_76
```

## Potential differences from existing files

- Extent to which we emulate the `isis_vms_compat` group to be discussed with Mantid team and other relevant consumers.

Start by reviewing the filewriter handling for the isis_vms_compat group and the listed VMS-name documentation at \\isis\shares\ISIS_Experiment_Controls\data for tickets\ds_76. Clarify the required behavior with the Mantid team and other consumers; done means an agreed compatibility scope for IRPB, NSP1, RRPB, RSPB, SPB, SPEC, and UDET.

Written by the indexing model from the issue text.