SNP — Added classes for describing bibliographic references

The model follows closely the OMG Bibliographic Query Service data
model. Only differences are the way bibref subject descriptors are
handles (giving more flexibility to mix different vocabularies) and
addition of boolean 'et_at' attribute to Person to indicate that the
list of, for example, authors is not complete.

• All the added classes except Bibref_subject and Bibref_description
  inherit from Identifiable and can have additional properties added to
  them using Annotation and Db_xref. These two classes are mere
  groupings of attributes to Bibliographic_reference and can not exist
  independently.

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