A struct for performing numerical integration by double-exponential (DE) formula.
It is also known as "Tanh-sinh quadrature".
In DE formula, the integration (1) is converted to (2).
(1) int_{xa}^{xb} f(x) w(x) dx
(2) int_{ta}^{tb} f(g(t)) w(g(t)) g'(t) dt
This module provides numerical integration by double-exponential (DE) formula.