--modality flag

[scovitz]

Allows users to only generate group outputs for a requested modality

[tsalo]

In BIDS, modalities are a mix of suffixes and datatypes:

Modality - The category of brain data recorded by a file. For MRI data, different pulse sequences are considered distinct modalities, such as T1w, bold or dwi. For passive recording techniques, such as EEG, MEG or iEEG, the technique is sufficiently uniform to define the modalities eeg, meg and ieeg. When applicable, the modality is indicated in the suffix. The modality may overlap with, but should not be confused with the data type.

I propose we implement a --datatype flag instead.