Speed up `ht.percentile` about heat

[mrfh92]

As far as I understand, computing percentiles in a distributed setting might really pose a problem in terms of quite heavy communication as re-ordering might be necessary. An option might be to add a faster variant using sketching, i.e. drawing a "small" sample from the data set and computing percentiles for this sample.

Maybe https://arxiv.org/pdf/2004.08604.pdf helps, although this algorithms has been designed for 1d (?) data streams.

[mrfh92]

I have implemented a version of ht.percentile that estimates the true percentile only on behalf of a random sketch of the entire data set. #1420

[github-actions]

Branch features/1389-Speed_up_ht_percentile created!