Allow multiple separate annotations

[luciansmith]

When translating an annotated model such as one from biomodels, the antimony version has elements like:

  MgATP part "http://identifiers.org/chebi/CHEBI:25107",
         "http://identifiers.org/chebi/CHEBI:15422"
  MgATP part "http://identifiers.org/kegg.compound/C00305",
         "http://identifiers.org/kegg.compound/C00002",
         "http://identifiers.org/chebi/CHEBI:18420"

However, when this is translated back into SBML, it is all added as a single annotation, and when re-translated to Antimony, you get:

  MgATP part "http://identifiers.org/chebi/CHEBI:25107",
         "http://identifiers.org/chebi/CHEBI:15422",
         "http://identifiers.org/kegg.compound/C00305",
         "http://identifiers.org/kegg.compound/C00002",
         "http://identifiers.org/chebi/CHEBI:18420"

Antimony should keep them separate.