Changes made in dsp/fourier/internal/fftpack break the formatting used
there, so these are reverted. There will be complaints in CI.
[git-generate]
gofmt -w .
go generate gonum.org/v1/gonum/blas
go generate gonum.org/v1/gonum/blas/gonum
go generate gonum.org/v1/gonum/unit
go generate gonum.org/v1/gonum/unit/constant
go generate gonum.org/v1/gonum/graph/formats/dot
go generate gonum.org/v1/gonum/graph/formats/rdf
go generate gonum.org/v1/gonum/stat/card
git checkout -- dsp/fourier/internal/fftpack
* diff/fd: implement Hessian finite difference, and code cleanups.
This commit primarily adds the Hessian function for finding a finite difference approximation to the Hessian. At the same time, it combines duplicated functionality across the difference routines so that the preludes to all the difference routines look similar
