Add 2D Gauss-Legendre quadrature integrator

[Mateasek]

I would like to add a GaussianQuadrature2D integrator. The implementation will be close to what @vsnever did in GaussianQuadrature class. It is related to issues #472 and #471 . I will separate the PR from #473 for clarity and to keep PR small.

[jacklovell]

Fixed in #476