Modification of string cast in owl_reader.py

[khelm]

I would like to suggest a modification to owl_reader.py. Right now any modifications to the nidm-experiment owl or import files fails due to the string cast here. The files pass the Travis checking if the cast to string is removed. This was discussed awhile ago, but it was never changed. IIRC the reason for the cast was not clear at the time. @cmaumet is there any reason not to make this modification?