A symmetry thorn with a 180 degree rotating symmetry. This will

replate PlanarPointSymmetry, which has a stupid name.


git-svn-id: http://svn.cactuscode.org/arrangements/CactusNumerical/RotatingSymmetry180/trunk@2 20f44201-0f4f-0410-9130-e5fc2714a787

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+CVS info   : $Header$
+
+Cactus Code Thorn RotatingSymmetry180
+Thorn Author(s)     : Erik Schnetter <schnetter@aei.mpg.de>
+Thorn Maintainer(s) : Erik Schnetter <schnetter@aei.mpg.de>
+--------------------------------------------------------------------------
+
+Purpose of the thorn:
+
+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.