Add a changelog entry.

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+Fixed: The VectorTools::integrate_difference() function allows users
+to provide a weight function that can also serve as a component mask
+to select individual components of the solution vector for error
+computation. For components not selected, such a mask would then
+simply be zero.
+<br>
+In some cases, the solution vector contains NaN numbers, for example
+when one uses the FE_FaceQ element for certain components of the
+solution vector and uses a quadrature formula for error evaluation
+that has quadrature points in the interior of the cell. For any
+"regular" solution component for which the component mask has a zero
+weight, the value of that component will be multiplied by zero and
+consequently does not add anything to the error computation. However,
+if the NaNs of a FE_FaceQ are multiplied with zero weights, the result
+is still a NaN, and adding it to the values times weights of the other
+components results in NaNs -- in effect rendering it impossible to get
+any information out of the VectorTools::integrate_difference()
+function if one of the finite elements involved is FE_FaceQ.
+<br>
+This is now fixed by simply skipping vector components for which the
+weight vector is zero. This has the same result as before for all
+"normal" situations, but also properly skips the NaN case outlined
+above.
+<br>
+(Wolfgang Bangerth, 2018/03/29)