Support RefSeq namespaces for searching

[reece]

SeqRepo currently uses NCBI as the namespace for what should be called RefSeq. We currently have NCBI in use in other biocommons packages, but that should not be further entrenched. To enable developers to use the proper RefSeq namespace, manually translate "RefSeq" to "NCBI" when finding accessions (i.e., SeqAliasDB::find_aliases()).

This action is a stop-gap until we address #31.