Function to process site ID value from pscis_crossing_id and my_crossing_reference
Source:R/fpr_tidy_assign_site_id.R
fpr_tidy_assign_site_id.Rd
Intended for processing field data from digital form in which only one value should be provided. It checks to confirm the dataframe has one and only one of either a `pscis_crossing_id` or a `my_crossing_reference`. It also uses [fpr_chk_dupes()] to issue a warning if there are duplicate values of `pscis_crossing_id`, `my_crossing_reference` or `site_id`