Add multiple threshold clustering to linker

[RossKen]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

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Adding linker.clustering.cluster_pairwise_predictions_at_multiple_thresholds to the linker to provide similar behaviour to linker.clustering.cluster_pairwise_predictions_at_threshold, including outputting node information alongside the cluster_ids (unlike the non-linker version).

Part of a wider piece of work to generate graph metrics for multi-threshold clustering (graph metrics requires a linker).

One decision point I would appreciate a second opinion on is the treatment of cluster_summary_stats. In the non-linker version there is a parameter to return summary statistics as opposed to the whole result. Here I have added the summary stats to a metadata dictionary (inspired by the Andy's graph metrics). I think this covers all bases, but open to any other suggestions. Or if there is a better way to structure the code to make it more streamlined.

PR Checklist

• Added documentation for changes
• Added feature to example notebooks or tutorial (if appropriate)
• Added tests (if appropriate)
• Updated CHANGELOG.md (if appropriate)
• Made changes based off the latest version of Splink
• Run the linter
• Run the spellchecker (if appropriate)

[RossKen]

FYI, still need to add tests but marking as ready for review to get feedback on the general concepts