Describe the new feature or enhancement

Support for fNIRS data acquired with three wavelengths, specifically from Shimadzu fNIRS devices.

Describe your proposed implementation

Using mne-python v0.24.dev0 and mne-nirs v0.1.2.dev, read_raw_snirf() will not load fNIRS data with 3 wavelengths (780, 805, 830) and expects only 2 wavelengths. The error message is "NIRS channels not ordered correctly. Channels must be ordered as source detector pairs with alternating frequencies: 780 & 805"

Using previous versions of mne-python (0.21.2) and mne-nirs (v0.0.1), it was possible to load the 3-wavelength data then to use drop_channels() to remove the channels associated with wavelength 830 before proceeding.

Here is some some sample data: sample_shimadzu_data.snirf

Thanks in advance!