Blurb::
Specify how to compute gradients and hessians

Description::
The \c interval_type setting is used to select between \c forward and
\c central differences in the numerical gradient calculations. The \c
dakota, DOT \c vendor, and OPT++ \c vendor routines have both forward
and central differences available, the CONMIN, NL2SOL and ROL \c vendor
routines support forward differences only, and the NPSOL and NLSSOL
\c vendor routines start with forward differences and automatically
switch to central differences as the iteration progresses (the user has
no control over this). The following forward difference expression
\f[
\nabla f ({\bf x}) \cong 
\frac{f ({\bf x} + h {\bf e}_i) - f ({\bf x})}{h}
\f]
and the following central difference expression
\f[
\nabla f ({\bf x}) \cong 
\frac{f ({\bf x} + h {\bf e}_i) - f ({\bf x} - h {\bf e}_i)}{2h}
\f]
are used to estimate the \f$i^{th}\f$ component of the gradient vector. 

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