File GtestMissingError.h added into Arithmetics/UnitTests.

/***************************************************************************

                          GtestMissingError.h  -  description

                             -------------------

    begin                : Jul 2, 2017

    copyright            : (C) 2017 by Tomas Oberhuber et al.

    email                : tomas.oberhuber@fjfi.cvut.cz

 ***************************************************************************/

/* See Copyright Notice in tnl/Copyright */

#pragma once

#include <stdexcept>

struct GtestMissingError

   : public std::runtime_error

{

   GtestMissingError()

   : std::runtime_error( "The GTest library is needed to run the tests." )

   {}

};