New appointment arguments

Please use this issue to suggest new arguments for [poli-sci-kit.appointment.methods](https://github.com/andrewtavis/poli-sci-kit/blob/main/src/poli_sci_kit/appointment/methods.py) functions if you know of a style of appointment variable that is not yet implemented. These new arguments can then be converted into good first issues that would include the necessary codes for implementation and testing.

Thanks for your interest in contributing!

Start by reading src/poli_sci_kit/appointment/methods.py and identify appointment-variable styles that are not implemented. This issue is a request for suggestions rather than a defined change; a concrete proposal would need to specify the new argument, implementation code, and tests before work is complete.

Written by the indexing model from the issue text.