Replacing llt with ldlt decomposition in the Laplace approximation

[spinkney]

Summary:

The Laplace approximation code at

stan/src/stan/services/optimize/laplace_sample.hpp

Line 83 in a3ee52e

Eigen::MatrixXd L_neg_hessian = (-hessian).llt().matrixL();

uses a Cholesky decomposition. In other parts of Stan's code base we use the ldlt decomposition from Eigen due to it's superior numerical stability (at the expense of being slower).

Description:

Replace the llt() with ldlt() for slower but more numerically stable code.

Current Version:

v2.31.0