filewriter: instrument group #97

Things under raw_data_1/instrument.

Potential differences from existing files

• source/type - propose to write strings that match the upstream nexus NXsource standard i.e. Spallation Neutron Source instead of Pulsed Neutron Source.
• source/probe - propose to write strings that match the upstream nexus NXsource standard i.e. neutron instead of neutrons.
• dae/detector_table_file: detector.dat doesn't exist in new system. We can either write a blank entry, or not write it at all.
• spectra_table_file/wiring_table_file: will point at a differently-formatted file, and this file will likely not be accessible from NDX. If the files are hosted centrally (e.g. git), we could point this at a git URL if needed. Does anything currently expect to be able to parse the files given in these filepaths?
• dae/time_channels_<N>/time_of_flight_raw & detector_<N>/time_of_flight_raw: data in cycles of a 32MHz clock. This doesn't make sense in streaming system. We can back-calculate it from data in ns if required. Do we need to write it?
• dae/vetos: There will be more vetos, and with different names, in the streaming system.
• dae/type: Will be something other than ISIS_DAE3. To-be-determined exactly what.
• dae/detector_<N>/polar_angle & dae/detector_<N>/distance: the streaming system currently doesn't know about geometry information. Do downstream consumers require these?