Gradient example patch

[Stephen Smith]

This patch adds Gradient.cpp in NewField/tests as an example code
that uses the nearest neighbor function.  Also, some minor fixes in
Field to get it working:
Field.h - made centering() accessor const
FieldCentering.h - added position(i) and orientation(i)
NearestNeighbor.h - added inputPosition function to compute a position
                    in logical coordinates from a field offset.
FieldShiftEngine.h, FieldStencilEngine.h, FieldEngineBase.h
  - switched from Pooma::NoInit to use default engine constructors, since
    that's currently supported by all engines.

Tested using KCC on Linux.  Reviewed by Jeffrey Oldham.

     Stephen