SplinkDataFrame metadata in clustering + metrics

[ADBond]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

Is your Pull Request linked to an existing Issue or Pull Request?

Closes #1971. It does not include saving metadata in parquet - will open a separate issue for this.

Give a brief description for the solution you have provided

SplinkDataFrame holds a dict attribute metadata which we can use to store whatever we want.

This stores threshold_match_probability on frames when we:

• cluster pairwise
• compute cluster metrics

In the latter we use the former metadata (if available) when no explicit threshold parameter is provided.

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

[RobinL]

LGTM 👍

[zslade]

🙌

[ThomasHepworth]

I've also scanned through this and it looks good to me.