`-bicol` option also overrides `-sicol` option

[rambaut]

By default the -sicol option uses column name sequence_name to match metadata to sequence id. But if you specify a different -bicol column to match your query then -sicol also changes. I don't think this is intuitive behaviour.

[rambaut]

Further to this perhaps it would be best to have an option -idcol which specifies the column to use for matching for all the options and then -sicol and -bicol (and -icol?) are used to override this for specific cases.