`filewriter`: periods

Entries under the `raw_data_1/periods` group.

Some of these entries are just per-period versions of dynamic metadata that exists elsewhere. These items should be straightforward to do it the same way as other 'dynamic' metadata.

Other items involve details of hardware-period schemes, which need discussion with DSG to work out how the new electronics treat these.

## Questions

HRPD-X will likely not use periods, but we need to do research to understand:
- How exactly does the streaming system handle hardware periods, DWELL periods, and suchlike (talk to DSG)
- Which elements of the current format make sense in streaming system?
- Which parts of the `raw_data_1/periods` group does mantid _minimally_ require to load a file? (talk to Mantid team)
- Which parts of the `raw_data_1/periods` _can_ mantid read for instruments that run in multi-period mode regularly? (talk to Mantid team)

## Potential differences from existing files

- Per-period metadata: no differences expected
- Hardware-period related datasets: to be determined, pending discussions with relevant teams about how hardware periods work in new system.

Start with the `raw_data_1/periods` group and separate per-period dynamic metadata from hardware-period datasets. Talk with DSG about hardware periods and with the Mantid team about the minimally required and readable elements. Done means the relevant period behavior and supported datasets are understood and documented for the streaming system.

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