Add support for BM25's k3 parameter to query parsers

### Description

Currently, duplicate terms in query strings see their scores boosted by their number of occurrences. It would likely be better to saturate the contribution of such repeated terms, as suggested in the original BM25 paper through the k3 tuning parameter.

From a quick survey, k3 seems to often default to 7 or 8, see e.g. Terrier: https://github.com/terrier-org/terrier-core/blob/54e6faf802bb6275b53ad6de14a93a53389719b8/modules/core/src/main/java/org/terrier/matching/models/BM25.java#L48.