Meta: Add interrogator type

[miili]

Add fields:

instrument_type: str
instrument_serial_number: str
author: str
campaign: str

[andreas-wuestefeld]

This is interesting information but not crucial for the geophysicist analysing the data
I suggest these things to go in the user-defined header
We could also suggest that the header follows this more complete model https://github.com/DAS-RCN/DAS_metadata

[miili]

Since it has been worked out in the above repo, why not use it here?

https://github.com/DAS-RCN/DAS_metadata/blob/main/whitepaper/DAS_metadata_model_15Aug2022.pdf

[andreas-wuestefeld]

Unfortunately this is still work-in-progress.
variable names have not been defined, readers and writers are not in place.