[BioC] rtracklayer gff import

[Cook, Malcolm]

> rtracklayer currently considers GFF3 files to be right-open. 
> The GFF3 spec
> states that start is always <= end, and that zero-width 
> intervals have start
> == end. 

yes but 1 width intervals also have start = end

> To me, this suggests that they are right-open. 
> Otherwise, you need
> some other way to distinguish zero vs. one width intervals, 
> which is crazy.

yes - it is crazy