Add merge option

[JackLiang1272]

Overview

Add merge option "first", "last", "nearest", and "count" on top of "forward".

Description of changes

Add merge option "first", "last", "nearest", and "count" on top of "forward".
"first" finds the first instance of the event with matching keys. This includes events happened before the predictions.
"last" finds the last instance of the event with matching keys.
"nearest" finds the nearest instance of the event with matching keys. This includes events happened before the predictions.
"count" creates a count column for each possible value of that event type.

Author Checklist

• Linting passes; run early with pre-commit hook.
• Tests added for new code and issue being fixed.
• Added type annotations and full numpy-style docstrings for new methods.
• Draft your news fragment in new changelog/ISSUE.TYPE.rst files; see changelog/README.md.

[CLAassistant]

All committers have signed the CLA.