Numerical integration over 3D domain of vector-valued function

[stevengj]

@Lisette_de_Bruin

For now, I want to ignore the fact that it is a singular integral and add a small number to the denominator.

That will still make Gaussian quadrature converge slowly until you have a large number of quadrature points, similar to the behavior described here with a near pole: (Insanely-) Higher Order Derivatives - #17 by stevengj