If the surfaceFieldValue function object is used to compute an
area-normal average or integral of a vector quantity, the result will
now be correctly written out as a scalar.
Previously surfaceFieldValue was limited to writing the same type as the
input field. A vector area-normal average or integral therefore had to
be written out as a vector. This was done by setting the x component to
the result, and the y and z components to zero. This was considered to
be counter-intuitive.
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