tanh-sinh quadrature

[jacobwilliams]

Consider adding fixed-precision versions of David H. Bailey's routines from tquad.f90 in MPFUN2020.

• quadts: tanh-sinh quadrature scheme of Takahashi and Mori, for functions on a finite interval such as (0,1).
• quades: exp-sinh scheme, a variation of tanh-sinh well-suited for functions on a semi-infinite interval such as (0, +infinity).
• quadss: sinh-sinh scheme, a variation of tanh-sinh well-suited for functions on the entire real line.

[ivan-pi]

The package DE-Quadrature also contains a Fortran implementation of tanh-sinh quadrature (also called double exponential quadrature).

@arjenmarkus also offered an implementation of his in fortran-lang/stdlib#612 (see also my comment therein).