Keep incomplete genes with complete features

With this change the genbank parser no longer discards incomplete genes
directly but keeps them as long as they have complete features
annotated.

For example, the PIK3R2 gene is annotated on NC_000019.9 (or a slice) as
4973..>22328 with two RNA entries. One of these, however, is complete so
it would be a shame to discard the entire gene.