convert_gff3_to_gbk.py, convert sequences with no annotation

[jonathancrabtree]

Currently convert_gff3_to_gbk.py will only create GenBank entries for input sequences that have at least one feature localized to them in the GFF. However, one might want to create GenBank entries for genomic sequences (in the FASTA section of the input GFF3) that have no features localized to them. The description of the converter ("Converts GFF3 representing gene models to Genbank flat-file format.") does suggest that the conversion process is based around gene models rather than sequences, but since the GenBank flat file format is inherently sequence-based it would be good to at least have an option to include unannotated sequences in the conversion.