Cactus Code Thorn CarpetEvolutionMask
Author(s)    : Christian D. Ott <cott@aei.mpg.de>
               Erik Schnetter <schnetter@cct.lsu.edu>
Maintainer(s): Christian D. Ott <cott@aei.mpg.de>
               Erik Schnetter <schnetter@cct.lsu.edu>
Licence      : GPLv2+
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1. Purpose

Provide a grid function "evolution_mask" which is zero for all grid
points that do not need to be evolved because they are going to be
restricted from a finer grid.