return GTF file format for MetaGenome object #196
Description
is_gtf argument in metagenome_to_gff is not effective
If I read code correctly, for MetaGenome, the export method will return the gff_handle_ref shock file directly and ignore the is_gtf argument.
https://github.com/kbaseapps/GenomeFileUtil/blob/master/lib/GenomeFileUtil/core/GenomeToGFF.py#L66