bids_export relies on session number - to fix #231
Description
where: there are a series of if statements starting @ https://github.com/sccn/EEG-BIDS/blame/master/bids_export.m#L507
expected behaviour: any label is valid for sessions since it is a string in the spec
identified issue: these relies on the length because it assumes number like ses-1 run-2 but one can have ses-baseline and run-1 (runs are ok since defined non negative indices)
possible fix have not figured out