Associate trait variables for NIRS spectra

[lukasmueller]

Each type of NIRS spectrum should have a trait variable associated with it, as a variable_of a generic NIRS_spectrum trait. The trait variable would be associated with a specific type of NIRS spectrometer, a protocol, and other metadata, such as repetitiveness or could also be a post composed trait

[keocorak]

Question - is it in scope of this ticket to also relate NIR variables to the variables predicted from the associated data? Possibly one to many.

[keocorak]

This is associated with issue 5918