The generic boundary interface

The implementation Boundary provides a number of aliased functions, which allow application thorns to register routines which provide a particular physical boundary condition, and also to select variables or groups of variables to have boundary conditions applied to whenever the ApplyBCs schedule group is scheduled (see section A1.3.3). In addition, an aliased function is provided which returns a list of grid variables selected for a boundary condition (see appendix A1.12).