ps_env_map family

[sebdalgarno]

currently, we have ps_env_map_df, but this should be generalized and should allow values to be returned (currently manipulates data.frames in-place)
could include ps_env_map_if/map_chr/map_df etc.
need to research what purrr is doing in this area and whether there are other functions we can build from (e.g. eapply)