Update gsl_integration.pxd

Added missing hook to a doubly-adaptive integration routine from GSL: gsl_integration_cquad. Namely:
1) ctypedef struct gsl_integration_cquad_workspace
2) gsl_integration_cquad_workspace_alloc (size_t n)
3) gsl_integration_cquad_workspace_free (gsl_integration_cquad_workspace * w)
4) gsl_integration_cquad (...)
Merge pull request #18 from AlexShevtsov/master

Add gsl_integration_cquad