make diagonal matrix cols() and rows() methods constexpr

Alex_Mrequested to merge
Alex_rath3t/eigen:fix/diagonalMatrix into master

What does this implement/fix?

This PR adds the Eigen constexpr macro EIGEN_CONSTEXPR to the cols and rows() methods of Eigen::DiagonalMatrix. This also lines it up with Eigen::Matrix which already has this constexpr methods. This is then inline with #2152 (closed).

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