Fixing for GCC8

src/FiniteDifference/eq_unit_test_3d.cpp

src/Operators/Vector/cuda/vector_dist_operators_cuda.cuh

...erators/Vector/tests/vector_dist_operators_tests_util.hpp

src/Operators/Vector/vector_dist_operators.hpp

src/Operators/Vector/vector_dist_operators_apply_kernel.hpp

.../Vector/vector_dist_operators_apply_kernel_unit_tests.cpp

...s/Vector/vector_dist_operators_apply_kernel_unit_tests.cu

src/Operators/Vector/vector_dist_operators_extensions.hpp

src/Operators/Vector/vector_dist_operators_functions.hpp

src/Operators/Vector/vector_dist_operators_unit_tests.cu

...lver<double>_lid_driven_cavity_3d_p1_grid_0_test_GCC8.vtk

...lver<double>_lid_driven_cavity_3d_p2_grid_0_test_GCC8.vtk

...lver<double>_lid_driven_cavity_3d_p2_grid_1_test_GCC8.vtk

...lver<double>_lid_driven_cavity_3d_p3_grid_0_test_GCC8.vtk

...lver<double>_lid_driven_cavity_3d_p3_grid_1_test_GCC8.vtk

...lver<double>_lid_driven_cavity_3d_p3_grid_2_test_GCC8.vtk