FieldOffset reductions patch.

[Stephen Smith]

This patch to newfield_revision allows data-parallel reductions using
field offsets.  The new syntax is:

g = sum(f, nearestNeighbor(f.centering(), g.centering()), g.centering());

which sums the points in f that are nearest to the points in g and assigns
the results to g.

I also fixed a problem with the domain of discontinuous fields by making
the translation between domains more explicit.

This was reviewed by Jeffrey, and tested on linux with KCC.

    Stephen

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21Aug.reduction.2.patch
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