Experiment setup() does not handle properly blocks with multiple outputs

Following the major revamping of this package, blocks with multiple outputs are not handled in the right way. In particular, the current implementation of the setup() method for Experiment assumes that a block has a single output (e.g. see https://gitlab.idiap.ch/biometric/beat.core/blob/v0.8.1/beat/core/experiment.py#L858). I am unsure whether there are other implications in the code or not.