bids_export relies on session number - to fix

[CPernet]

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