Pairwise string distance comparison

[zmbc]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

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

This is a follow up to #2195, addressing the PR comments there. Closes #1994.

Give a brief description for the solution you have provided

As discussed in the prior PR, this mostly models PairwiseStringDistanceFunctionAtThresholds and PairwiseStringDistanceFunctionLevel off of DistanceFunctionAtThresholds and DistanceFunctionLevel
respectively.
The main difference is that it is pairwise on an array column (duh) and that it only accepts a small list
of string distance functions and transpiles them, instead of the user passing an arbitrary SQL function.

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)

[RobinL]

Thank - I think this looks good. Minor comment below about the default argument and suggested refactor of test to align to the newer format - but other than that i think this is good to merge

[zmbc]

@RobinL I believe I've addressed your comments. I don't understand why a test is failing -- it does not seem related in any way to these changes.

[ADBond]

@RobinL I believe I've addressed your comments. I don't understand why a test is failing -- it does not seem related in any way to these changes.

@zmbc you are correct - apologies this is an unrelated issue #2515 (which will be fixed shortly, so should not be an issue going forward). Have re-run it to get it to pass, for clarity.

[RobinL]

Brilliant, thanks @zmbc and @JonnyShiUW this is great