[ENH] Use Label encoder for site handling

[oesteban]

https://github.com/poldracklab/mriqc/blob/8c2e33533637e53bbd5488b8c2ee84c7138a5fe8/mriqc/classifier/sklearn/preprocessing.py#L128
https://github.com/poldracklab/mriqc/blob/8c2e33533637e53bbd5488b8c2ee84c7138a5fe8/mriqc/classifier/nested_helper.py#L169
https://github.com/poldracklab/mriqc/blob/18c40d43d0324f4653d3ca540fb90185a1b46af9/mriqc/classifier/helper.py#L534

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.