humann: use versioned metaphlan cached reference

[bernt-matthias]

and also allow to use legacy data.

We have to do this because the old datatable may contain entries that are not usable without the --mpa3 flag. Alternatively we could filter legacy entries.

I could also imagine to add a boolean flag allowing for legacy metaphan data from the history, but I don't care so much.

ping @tuncK

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