Estimate Hessian using triangular subsitution algorithm

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`Y` |
Matrix of finite differences of gradients |

`colors` |
Vector of length nvars that identifies color of each variable |

`jCol, ipntr` |
Column indices and row pointers for non-zero elements of lower triangle of Hessian (row-oriented compressed format). |

`delta` |
Perturbation factor used to compute finite differences of gradients. |

`nvars` |
Dimension of Hessian (number of variables) |

`nnz` |
Number of non-zero elements in the lower triangle of the Hessian. |

For internal use. You should not have to call this function directly.

A sparse Hessian of class dgCMatrix.

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