psychsyn code review

- psychsyn uses internal helpers, `get_item_pairs` and `syn_for_one`. What is the best way/place to put these helpers? Right now we have them at the end of psychsyn.R
- potentially replace `aperm()` with `t()` as it produces identical results (verify) and is more well known, or else edit syn_for_one so that its output is shaped correctly. 
- write tests