SampleSet missing attributes 'key'

With the updated version of the CSVDatabase the references and probes are now SampleSet objects. Special care is taken to make sure the required attributes are defined for samples within them.

However there is no check for SampleSet attributes, which is not directly an issue. But later down the pipeline, when using checkpointed experiment, the BioAlgCheckpointWrapper rely on the key attribute to run (i.e. _enroll_sample_set(...)).