Improvements to the test for balanced growth path

— Raise the default tolerance for cross-derivatives to 1e-6, to reduce the
  number of false positives

— New option “balanced_growth_test_tol” to the “model” block for changing that
  tolerance

— Turn back test failures into errors. Since there is now an option for
  controlling the tolerance, the user always has the possibility of making the
  test pass.

Closes: dynare#1389
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