Control to Treatment Matching

[spohngellert-o]

Proposed changes

Add a parameter to NearestNeighborMatch that allows control to treatment matching. This is useful in scenarios when the control set is much smaller than the treatment set.

Types of changes

What types of changes does your code introduce to CausalML?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

The changes here somewhat change variable conventions in match, and so I am open to changing any variable names/conventions. Please let me know if there are any questions or comments.

[jeongyoonlee]

LGTM. Thanks for your continuous contribution and support!