code: def add_creator(self, doc, creator):
          
          if validations.validate_creator(creator):
              doc.creation_info.add_creator(creator)
              return True
          else:
              raise SPDXValueError('CreationInfo::Creator')
docs: Adds a creator to the document's creation info.
          Returns true if creator is valid.
          Creator must be built by an EntityBuilder.
          Raises SPDXValueError if not a creator type.