Create residual viscosity package +rv and generalize the ResidualViscosity class - Generalize residual viscosity, by passing user-defined flux and calculating the time derivative outside of the update. - Create separate RungekuttaRV specifically using interior RV updates - Separate the artifical dissipation operator from the scheme AdvectionRV1D so that the same scheme can be reused for creating the diff op used by the ResidualViscosity class

A grid.boundaryIdentifier identifies a boundary of a grid.
For a Cartesian grid it is simply 's', 'n', 'w', 'e'.
For a multiblock grid it might be something like {1, 's'}.
For some other grid it will be up to the developer to chose
a good way to identify boundaries.