Fixing numerics on GCC6

src/FiniteDifference/eq_unit_test.hpp

src/Solvers/petsc_solver.hpp

test/AMG_psi_0_test_GCC6.vtk

test/AMG_psi_1_test_GCC6.vtk

test/AMG_psi_2_test_GCC6.vtk

test/AMG_psi_test_0.vtk

test/AMG_psi_test_1.vtk

test/AMG_psi_test_2.vtk

...lver<double>_lid_driven_cavity_3d_p1_grid_0_test_GCC6.vtk

..._solver<double>_lid_driven_cavity_p1_grid_0_test_GCC6.vtk

..._solver<double>_lid_driven_cavity_p3_grid_0_test_GCC6.vtk

..._solver<double>_lid_driven_cavity_p3_grid_1_test_GCC6.vtk

..._solver<double>_lid_driven_cavity_p3_grid_2_test_GCC6.vtk