Cactus Code Thorn RotatingSymmetry180
Author(s)    : Erik Schnetter <schnetter@cct.lsu.edu>
Maintainer(s): Erik Schnetter <schnetter@cct.lsu.edu>
Licence      : GPL
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1. Purpose

Provide a 180 degree rotational symmetry boundary condition.  This is
the kind of symmetry that exists e.g. in a binary black hole evolution
when the two black holes are identical.  The same setup also admits an
additional bitant symmetry.

Currently the symmetry is fixed to be located at the lower x boundary,
so that there is a rotation about the z axis.

The coordinate thorn does not know about this symmetry.  You will have
to set up the domain such that you have an upper bitant in the x
direction.

This symmetry boundary condition can be mixed with arbitrary symmetry
conditions in the z direction, e.g. bitant or periodicity.