Gff-version directive
From SO Wiki
I modified the spec to explicitly state that:
"The GFF version, always 3 in this spec. This directive must be present, must be the topmost line of the file and may only appear once in the file"
The rationale is that it doesn't really make sense to declare the gff-version multiple time in a file as this doesn't change throughout a file.
I think some users have suggested that they use multiple gff3-version pragmas to create multi-organism GFF files, but I think this makes the GFF3 implementation too complex.
Barry 2/2/12
