Fix clt order

[mgiulini]

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Closes #580 by implementing the sorting logic in capri_cluster_analysis.

IMPORTANT: the rank in the caprieval table may differ from the rank coming from the clustfcc module, for instance when a different clt_threshold is defined. This is why I added a caprieval_rank field to the capri_clt table, which indicates the ranking according to the standard capri order (average of the score over the first clt_threshold models). Of course when a different sorting key (dockq, fnat, ...) is provided, the order changes and caprieval_rank remains unaltered (as in the single structure case).

Additionally, the cluster_analysis test has been extended