`filewriter`: Neutron histogram data (`raw_data_1/detector_1`)

The file-writing infrastructure must be able to write out histogram-mode data (`.nxs` files _without_ an `NXevent_data` group).

This refers to the NeXuS groups under `raw_data_1/detector_1`.

The issue is intentionally phrased as

> file-writing infrastructure

because this does not _necessarily_ have to be done in the same process as the event-mode filewriter (though having it in the filewriter would be more performant and potentially more flexible in future).

## Questions

- Support streamed histograms (`hs00`) at some point in future? i.e. a scheme where a monitor emits a histogram every _N_ frames, where N could be 1 in the extreme case. This then lets a run be sliced a bit like event mode, but only on N-frame boundaries.
* This _might_ be needed for SANDALS2 if monitor event-rates are too high for pure-event mode.
* Need to define how we would write this kind of data to `.nxs`.

## Approaches

- This could be an option in the filewriter process itself (preferred). It would then need various options to cope with:
* event files only
* combined histogram+event files
* separate histogram and event files
- This *could* be implemented as a separate process which reads in an event-mode `.nxs` file and outputs a histogram-mode `.nxs` file.
* But that then makes it harder to support all combinations of files we may need to generate, e.g. event-only, histogram-only, event+histogram in the same file, event-detectors with histogram-monitors, etc.
* This may also have a performance disadvantage as all of the events need to be written by one process, then read back in to another process before histograms can be generated. For large files this file-loading cost may be significant, especially for beamlines that ultimately only care about the histogram files.

## Potential differences from existing files

None known at this time.

The issue names no files or tests; start by locating the existing event-mode filewriter and the NeXuS handling for raw_data_1/detector_1. Clarify the supported histogram/event combinations and streamed-histogram expectations, then verify that the selected approach produces the required histogram-mode .nxs output.

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