Allow specifying a list of wavelengths

[DominicOram]

If we are running with a polychromatic beam we do not have a single wavelength but instead a list. The MXNX standard allows for this (https://manual.nexusformat.org/classes/base_classes/NXbeam.html#nxbeam-incident-wavelength-field) so we should accept it

[phyy-nx]

Hi, if it helps, this data shows per-shot wavelength.
https://zenodo.org/records/7011530

import h5py
f = h5py.File("run_000795.JF07T32V01_master_spectrum_4img.h5")
f['entry/instrument/beam/incident_wavelength'][()]
array([1.30777731, 1.30774953, 1.30824259, 1.30727774])

It also includes per-shot spectra as well.