Blurb::
Turn on regression diagnostics

Description::
When ``regression_diagnostics`` (NL2SOL's ``rdreq``) is specified and
a positive-definite final Hessian approximation H is computed, NL2SOL
computes and prints a regression diagnostic vector RD such that if
omitting the i-th observation would cause alpha times the change in
the solution that omitting the j-th observation would cause, then
:math:`RD[i] = |alpha| RD[j]`. The finite-difference step-size
tolerance affecting H is
:dakkw:`responses-numerical_hessians-fd_step_size`
(NL2SOL's ``delta0`` and ``dltfdc``).

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