Cactus Code Thorn RotatingSymmetry90
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 90 degree rotational symmetry boundary condition.  This is
the kind of symmetry that exists e.g. in a single black hole evolution
when the black hole has spin.  The same setup also admits an
additional bitant symmetry.

Currently the symmetry is fixed to be located at the lower x and y
boundaries, 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 quadrant in the x and
y directions.

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