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
Before the Digamma function could be quite slow for large negative
arguments because it had to do use the recurrence relation thousands
of times. This avoids that by instead using the reflection formula. It
also adds some explicit checks for special cases (Inf, NaN, poles).
