Complete NXcanSAS applicaiton definition

[jl-wynen]

The current implementation is minimal. We need more in order to save the relevant metadata to the file. See also scipp/esssans#33

[SimonHeybrock]

Can you elaborate what is needed here? Not everything can be automatic, so it is not clear what expansion/completion you have in mind.

[jl-wynen]

E.g., SASinstrument, SASsource, SASprocess. I didn't look in detail how they differ from their NX* counterparts. But we should at least have a wrapper to make sure that everything uses the SAS* classes.