Cactus Code Thorn Boundary
Authors: Gabrielle Allen, Gerd Lanfermann, Miguel Alcubierre
CVS info   : $Header$
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1. Purpose of the thorn

This thorn provides some standard outer boundary conditions, and contains
interfaces for applying boundary conditions either in all coordinate
directions, or individual coordinate directions.

Currently:

  - scalar boundary conditions

  - flat boundary conditions (a copy of the point just inside the
    boundary)
 
  - copy boundary conditions 

  - radiation boundary conditions

  - Robin boundary conditions


2. Additional information

This thorn only currently works with a 3D Cartesian Grid.

The boundary conditions can be called from Fortran or C, 
and can be passed either groups of grid functions, or single
grid variables.

All the boundary conditions work with an arbitrarily size stencil width, 
except for the radiation and Robin boundary conditions which currently only
work with a stencil width of one.

Radiation and Robin boundary conditions are only implemented for 3D grid
functions currently.