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

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.