SBML export annotations

[pcarlip]

It looks like the package lost the ability to export annotations to SBML from c0763d8, possibly during the conversion from manual xml generation to using libSBML. I believe libSBML supports annotations to at least some degree (see RDFAnnotationParser, CVTerm, add CVTerms example) so it should be possible to add that back in.

[jmuhlich]

Thanks for pointing this out and linking to the relevant libSBML API!