Speed up Bootstrapping Computation (#409)
* Added fix for heavy recomputation of sample level metrics.
* Moved parallelization to where it is actually useful.
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Co-authored-by: Clémentine Fourrier <22726840+clefourrier@users.noreply.github.com>
Co-authored-by: Nathan Habib <30601243+NathanHB@users.noreply.github.com>
