Cactus Code Thorn SpaceMask
Authors    : Tom Goodale
CVS info   : $Header$
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Purpose of the thorn:

This will provide two integer grid functions

space_mask
space_table_field

the first one will be a bitmask describing the point, with one bit
indicating that there is extra data in a table the handle of which can be
found in the corresponding point in the space_table_field.

Thorns interested in the mask should inherit from this implementation.

There may also be aliased query functions provided.