QAWC adaptive integration for Cauchy principal values

Function: int gsl_integration_qawc (gsl_function *f, double a, double b, double c, double epsabs, double epsrel, size_t limit, gsl_integration_workspace * workspace, double * result, double * abserr)

This function computes the Cauchy principal value of the integral of @math{f} over @math{(a,b)}, with a singularity at c,

The adaptive bisection algorithm of QAG is used, with modifications to ensure that subdivisions do not occur at the singular point @math{x = c}. When a subinterval contains the point @math{x = c} or is close to it then a special 25-point modified Clenshaw-Curtis rule is used to control the singularity. Further away from the singularity the algorithm uses an ordinary 15-point Gauss-Kronrod integration rule.