Centralize RDF/XHTML keywords (dc, Bag, rdf, etc.) as constants/enums

[dyrpsf]

This is a follow-up to PR #285 / issue #219.

During review, it was suggested to move repeated RDF/XHTML keywords such as
dc, Bag, rdf, etc. into a shared place, either as public static final
string constants or as an enum, to reduce the risk of typos and improve
long-term maintainability.

A possible approach would be to introduce something like:

• org.sbml.jsbml.xml.RDFConstants (for RDF, Bag, li, about, …),
• or an enum containing the commonly used element/attribute names,

and then have SBMLRDFAnnotationParser and related classes use these
constants instead of raw string literals.

This would be a pure code-style refactoring with no change in behavior.

I’d be happy to work on this if approved.

[dyrpsf]

Hi @draeger ,

I have opened PR #289 to address this.

The fix introduces a centralized RDFConstants class to manage repeated string literals (like dc, Bag, rdf, and li) and updates the SBMLRDFAnnotationParser to utilize these constants. This should resolve the risk of typos and improve the maintainability of the RDF parsing logic.