ENH: option to check merge is one-to-one, many-to-one, one-to-many, or many-to-many

ENH: option to check merge is one-to-one, many-to-one, one-to-many, or many-to-many: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). With that in mind, I'd like to propose adding a check merge argument that accepts strings one to one , one to many , may to one …