Fix GeneProduct cloning to copy required label and associatedSpecies

[dyrpsf]

Summary

This PR fixes sbmlteam/jsbml#252.

Previously, the GeneProduct(GeneProduct nsb) copy constructor only called the
super-constructor and did not copy the label (required) or associatedSpecies
fields. Since clone() is implemented as:

@Override
public GeneProduct clone() {
  return new GeneProduct(this);
}

cloning a GeneProduct would result in a cloned instance missing these fields,
potentially producing invalid SBML.

The copy constructor is now:

public GeneProduct(GeneProduct nsb) {
  super(nsb);

  if (nsb.isSetLabel()) {
    setLabel(nsb.getLabel());
  }

  if (nsb.isSetAssociatedSpecies()) {
    setAssociatedSpecies(nsb.getAssociatedSpecies());
  }
}

so that both label and associatedSpecies are preserved when cloning.

Verification
Local compilation with Maven (core + fbc) succeeds using Java 11.
Cloning a GeneProduct now yields an equal instance with the same label
and associatedSpecies.

Fixes #252