Session is always required

[akhanf]

For cross-sectional datasets we wouldn't want to use the session entity, or at the minimum, set it to a constant (I think the latter is possible maybe via remapping??)

But would be good to refactor the code to make session optional. Using the bids() function from snakebids is a good starting point, and perhaps using a conditional to set a subj_wildcards dict, eg. {'subject':'{subject}', 'session':'{session}'}, or {'subject':'{subject}'}, depending on whether the global no-session flag is used. Would need conditionals in a number of other spots too, as well as buried in the python scripts..