`filewriter`: specify file layout in detail

We need to formally document what we expect any 'new' .nxs files to look like, and pay particular attention to differences with the old system.

The goal is to be compatible where possible / where it makes sense, but some information may fundamentally differ between ICP and streaming stack, e.g.:

• Much of the information in the isis_vms_compat group
• How we write per-frame veto state and enablement alongside the event-mode data
• Various metadata fields
• Some pieces of information which only make sense in ICP (or in the newer streaming system)

This ticket must include discussion with stakeholders such as DSG and Mantid (and other relevant groups)

Acceptance criteria

• Documented structure for what a 'new' nexus file will look like
• Documentation describing differences where they exist (effectively a 'porting guide' between ICP and streaming files)
• An example file in the new format (does not need to have been written by 'new' filewriter; e.g. a simple python program can be used to generate this example file / manipulate an existing file)