Create a better itnerface to construct upwind DG operators

[ranocha]

Right now, one can do something along the lines of

D_local = LegendreDerivativeOperator(-1.0, 1.0, 3) # 4 nodes = polynomial degree 3
mesh = UniformPeriodicMesh1D(; xmin, xmax, Nx = 2^5)
D1 = PeriodicUpwindOperators(
    couple_discontinuously(D_local, mesh, Val(:minus)),
    couple_discontinuously(D_local, mesh, Val(:central)),
    couple_discontinuously(D_local, mesh, Val(:plus))
)

However, it would be nice to have a better API based on upwind_operators, e.g., something like

D1 = upwind_operators(D_local, mesh)

The basic question is whether this is the best option or whether passing something like couple_discontinuously would be better.