A decorator for other MFDB attributes to file NAs into another group, either one created by the main function or not.
mfdb_aggregate produced by another function, e.g.
The group to assign NAs to, e.g. "len_unknown"
The NA group won't be added to any aggregate files generated by MFDB, since the output would be invalid.
mfdb_aggregate object that can then be used in querying functions such as
length <- mfdb_na_group(mfdb_step_interval("len", 10), 'len_unknown')