Locate other performance bottlenecks, document, and add testing constraints

[gwaybio]

Discussed in https://github.com/cytomining/pycytominer/issues/195#issuecomment-1217265110

Add Memory/Time Testing Constraints for .merge_single_cells() (or others! 🙂 ) (Something like pytest-memray or similar): automate targeted performance tests to ensure operations within this project are usable by others as appropriate. Maybe this one spot was an outlier, but if there are other performance-heavy functions we could expand there as well.