Different numerical derivative implementations.

This namespace contains a discussion of polynomial interpolation, and different
methods for fitting a polynomial of degree N-1 to N points and evaluating that
polynomial at some different `x`.

This namespace contains a discussion of rational function interpolation, and
different methods for fitting rational functions N points and evaluating them
at some value `x`.

Richardson interpolation is a special case of polynomial interpolation; knowing
the ratios of successive `x` coordinates in the point sequence allows a more
efficient calculation.

Implements utilities shared by all integrators, for example:

- code to wrap a sequence of progressively better estimates in a common `integrator` interface
- data structures implementing various integration intervals.

## U Substitution and Variable Changes

This namespace provides implementations of functions that accept an
`integrator` and perform a variable change to address some singularity, like
an infinite endpoint, in the definite integral.

The strategies currently implemented were each described by Press, et al. in
section 4.4 of ['Numerical
Recipes'](http://phys.uri.edu/nigh/NumRec/bookfpdf/f4-4.pdf).

Trapezoid method.

This namespace contains an implementation of Brent's method for finding the
minimum of a real-valued function.