GFF3 files with sequence included

[tseemann]

In bacterial genomics we normally want the annotations and the sequences/contigs. This tool currently only handles each separately (as that is how NCBI provides it) via -F fasta and -F gff3.

This -F fullgff proposal would download the fasta and gff and construct the following:

##gff-version 3
<insert GFF>
##FASTA
<insert FASTA>

See https://github.com/The-Sequence-Ontology/Specifications/blob/master/gff3.md

[kblin]

That's where I'd use GenBank files, to be honest. Those have more data in then than the GFF files provide for all I can see from my ncbi-genome-download experience.
I see the appeal of this feature, but I need to think about how to integrate it in the current structure of the code.