This function is used to aggregate abundance data into one column (pct_cover). Abundance can come from four columns: "akstratumcoverclasspct", "speciescancovpct", "speciescomppct", and "understorygrcovpct". If the same record uses multiple abundance columns, pct_cover is calculated as the average of those values. When only one abundance column is used for a record, that value is used for pct_cover.