Add changelog

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+Improved: The constructor of MappingQGeneric used a naive inversion of a cell
+Poisson matrix, which became very slow for high approximation degrees in
+3D. The implementation has been switched to the fast diagonalization method
+implemented by TensorProductMatrixSymmetricSum.
+<br>
+(Martin Kronbichler, 2017/07/11)

diff --git a/doc/news/changes/minor/20170711MartinKronbichler b/doc/news/changes/minor/20170711MartinKronbichler
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+New: A new class TensorProductMatrixSymmetricSum that represents a tensor
+product of a mass and derivative matrix has been added. It implements both the
+matrix-vector operation (TensorProductMatrixSymmetricSum::vmult()) as well as
+the inverse operation (TensorProductMatrixSymmetricSum::apply_inverse())
+operation in tensorial form with optimal complexity of
+<tt>size<sup>dim+1</sup></tt> operations. The inverse operation uses the fast
+diagonalization method.
+<br>
+(Martin Kronbichler, 2017/07/11)