Read off the header rather than assumed, so a file counting per 100 cubic
metres is handled without being told. The most common suffix wins when a file
carries more than one, which warn_mixed_units() reports separately.
Value
the suffix, including its leading underscore; "_10M2" when the
header has none, since that is what the ECOMON export uses
Examples
raw_abundance_suffix(c("STATION", "CALANUS_FINMARCHICUS_100M3"))
#> [1] "_100M3"