[Biopython] GFF.writer

[Brad Chapman]

Peter;

> Would using an OrderedDict be neater? i.e. Preserve any user
> given order or whatever there was when parsing. This would
> allow ad-hoc conventions like the ID is first to be observed
> (or whatever the user preferred).

The current API generates the GFF from Biopython Seq and SeqFeature
objects, so there isn't a clean way to pass through ordering like this.
We could expose qualifiers as OrderedDicts if that's a useful change,
but still need to pick an ordering for non-qualifier items.

Practically, there is no guaranteed order to GFF3 attributes. Exposing
an alphabetized list seems reasonable but it's probably not worth going
too far down this path.

Brad