Add tests from Nektar++, fix bugs in implementation

ADRSolver/CMakeLists.txt

ADRSolver/EquationSystems/UnsteadyAdvection.cpp

ADRSolver/EquationSystems/UnsteadyAdvection.h

ADRSolver/Tests/Advection1D_WeakDG_GAUSS_LAGRANGE.tst

ADRSolver/Tests/Advection1D_WeakDG_GLL_LAGRANGE.tst

ADRSolver/Tests/Advection1D_WeakDG_MODIFIED.tst

ADRSolver/Tests/Advection2D_benchmark.xml

...ts/Advection2D_dirichlet_regular_GAUSS_LAGRANGE_10x10.tst

...Advection2D_dirichlet_regular_GAUSS_LAGRANGE_10x10_MF.tst

...ts/Advection2D_dirichlet_regular_MODIFIED_triangle_98.tst

...Advection2D_dirichlet_regular_MODIFIED_triangle_98_MF.tst

ADRSolver/Tests/Advection2D_m12_DG_quad_VarP.tst

ADRSolver/Tests/Advection2D_m12_DG_tri_VarP.tst

ADRSolver/Tests/Advection3D_DG_prism.tst

ADRSolver/Tests/Advection3D_m10_DG_hex_VarP.tst

ADRSolver/Tests/Advection3D_m10_DG_hex_periodic_nodal.tst

ADRSolver/Tests/Advection3D_m10_DG_prism_VarP.tst

ADRSolver/Tests/Advection3D_m12_DG_hex_periodic.tst

ADRSolver/Tests/Advection3D_m12_DG_prism.tst

ADRSolver/Tests/Advection3D_m12_DG_tet.tst