Re-enable parallelization in ``apply()``

[oesteban]

One of the things I knowingly let drop from the apply() outsourcing (#195) was the parallelization of resampling.

@jmarabotto - do you think this is something you can try to address this last week? I need to make progress fast because MRIQC has been suffering from memory issues since we changed this -- meaning, I'll try to be more online (and perhaps meet later today or tomorrow?)