BIDS incompatible with runs descriptions

[PierreGtch]

I missed it during implementation but the runs are expected to be indexes in BIDS (see https://bids-specification.readthedocs.io/en/stable/appendices/entities.html#run).
But in a few datasets (such as Lee2019), run indices are followed by a description string (like 1train or 2restingState). This makes it impossible to save these datasets in cache rn:

ValueError: run is not an index (Got 1train)

A possible solution would be, when saving such datasets, to split the run name, and encode the index part as run index and the string part as another entity field. But the problem is that no field perfectly matches this need. The best is probably the task field IMHO but we already use it to specify the paradigm.

I would argue we don’t actually need to specify the paradigm because, in every case, the whole dataset uses the same paradigm.