Clarify XML format for xml2gff

[lachjames]

Hi :)

I was hoping it might be possible to develop an XML schema for what is expected by xml2gff, as it appears to be quite picky (especially regarding the string formats). I could attach an initial draft of such a scheme to this issue in a comment in the next day or two - @DrMcCoy would you be willing to verify this and help me fix any errors in the schema?

Thanks :)

[DrMcCoy]

Oh, yeah, that would be useful, thanks.

If you want to try your hand on a less complex format first, xml2ssf and xml2tlk would be the candidates.