A preconditioner based on the digonal entries.
This class allows to approximately solve for A.x = b problems assuming A is a diagonal matrix. In other words, this preconditioner neglects all off diagonal entries and, in Eigen's language, solves for:
A.diagonal().asDiagonal() . x = b
| _Scalar | the type of the scalar. | 
This class follows the sparse solver concept .
This preconditioner is suitable for both selfadjoint and general problems. The diagonal entries are pre-inverted and stored into a dense vector.
    © Eigen.
Licensed under the MPL2 License.
    https://eigen.tuxfamily.org/dox/classEigen_1_1DiagonalPreconditioner.html