def get_statements(subject: nil)
      raise TriplestoreAdapter::TriplestoreException.new("#{@provider.class.name} missing get_statements method.") unless @provider.respond_to?(:get_statements)
      @provider.get_statements(subject: subject)
    end
def get_statements(subject: nil)
      raise TriplestoreAdapter::TriplestoreException.new("#{@provider.class.name} missing get_statements method.") unless @provider.respond_to?(:get_statements)
      @provider.get_statements(subject: subject)
    end
Get statements from the server
 @param [String] subject url
 @raise [TriplestoreAdapter::TriplestoreException] if the provider doesn't implement this method