`filewriter`: intermediate files & autosave files

The filewriter must support intermediate files - i.e. a request to write all of the data so far in the run to a `.nxs` file, without ending the run.

It must also similarly support autosave files - as above but happens automatically at intervals.

This is especially important as HRPDX plan to run their analysis in histogram mode, without these intermediate files a detector failure 11 hours into a 12 hour run would trash the 11 hours worth of data, with these files at least some data should be recoverable (this is not an issue for event mode, only histogram mode).

Start by locating the filewriter component and the request path that writes the current run data to a .nxs file. Check how histogram-mode output is handled, then define done as supporting an explicit intermediate-file request and periodic autosave without ending the run, with recoverable data after a detector failure.

Written by the indexing model from the issue text.