Jacobi preconditioner for LeastSquaresConjugateGradient.
This class allows to approximately solve for A' A x = A' b problems assuming A' A is a diagonal matrix. In other words, this preconditioner neglects all off diagonal entries and, in Eigen's language, solves for:
(A.adjoint() * A).diagonal().asDiagonal() * x = b
| _Scalar | the type of the scalar. | 
This class follows the sparse solver concept .
The diagonal entries are pre-inverted and stored into a dense vector.
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    https://eigen.tuxfamily.org/dox/classEigen_1_1LeastSquareDiagonalPreconditioner.html