`filewriter`: copy information from `instrument_components.nxs`

MARI nexus files contain a different set of datasets than other neutron instruments.

These 'special' nexus files are not needed for HRPD-X or SANDALS-2, but I'd like to take these requirements into account when designing the filewriter to ease future implementation when we need to move MARI to datastreaming at some point in the future.

This mechanism appears to correspond to https://github.com/ISISNeutronMuon/ISISICP/pull/39 so @mducle may be able to comment on the underlying need. The nexus file this information is copied _from_ is `\\ndxmari\c$\Data\instrument_components.nxs`.

On MARI specifically, this causes various information to get added to the generated `.nxs` for each run:
- `instrument/aperture`
- `instrument/fermi` exists and contains metadata from the fermi chopper including rotation speed, slit parameters, etc.
- `instrument/moderator` contains more information than other beamlines: it contains a pulse shape distribution, moderator type, temperature, transforms.
- `instrument/source` contains different information compared to a regular nexus file, including `frequency` and `target_material` but excluding `probe`
- Various geometry information is added, in a similar way to what was previously possible with `detector.dat`. My understanding is that MARI do actually use this geometry information in preference to the Mantid IDF.

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## Questions

- Would an ESS-filewriter style 'nexus layout' structure be sufficiently flexible to cater for this need - I think it should be? But it might tie us into a more complicated architecture than we might otherwise need...

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Note that MARI is not on any near-term datastreaming list that I've seen, so we can indefinitely defer actually implementing this functionality. This issue exists primarily for requirements-gathering.

Start by examining the MARI source file `instrument_components.nxs` and the linked ISISICP pull request 39, then compare the listed MARI datasets with the existing ESS-filewriter-style nexus layout. This issue is complete when the requirements and an agreed flexible layout for aperture, fermi, moderator, source, and geometry information are documented; implementation is explicitly deferred.

Written by the indexing model from the issue text.