Serialization does not produce a valid SBML

There is no `xml` header and _probably_ the namespaces in the `` are wrong. This makes it impossible to read models serialized with rust_sbml through libSBML, which is a big flaw.

There are other problems that may make a model readable through rust_sbml but not libSBML like identifier uniqueness through `Species` and `Reaction` that rust_sbml does not check at all. There may opportunities for validation/automatic correction in the future.