Fix ambiguous DiagonalMatrix constructors.

The following became ambiguous:

const Eigen::DiagonalMatrix<double, 4> m({1, -1, -1, 1});

since the initializer list {1, -1, -1, 1} could create either a DiagonalMatrix with list of scalars, or a DiagonalVectorType with list of scalars. Added a single initializer list constructor to avoid this ambiguity.