Implementation of Galerkin Quadrature methods and additional tutorials

[Aggie04]

This PR will add the Galerkin Quadrature methods 1 and 3 into OpenSn to offer another option when forming the scattering source. These scattering operators are formally expressed in literature.

In addition to this, 5 tutorials have been added --
Triangular quadrature sets and Lebedev quadrature sets have been given tutorials that are inline with the Product and SLDFE tutorials, explaining the requirements for them and an example of their angular grid for a given order.
operator_methods.ipynb exists as an introduction to the different ways the discrete-to-moment flux maps can be constructed. A quick aside is also mentioned here that the D2M matrix is stored as the transpose of the literature-defined D2M.
Two tutorials (one with highly forward peaked Henyey-Greenstein scattering, the other with pure mu=1 delta scattering) were added. The first one shows that a large cross section expansion is needed to converge the legendre moments, but even a truncated expansion is capable of yeilding good results if the solution is well represented by harmonics. The second tutorial exists to showcase the major differences between the Standard method and the Galerkin-Quadrature methods, which differences are exacerbated in highly-forward peaked problems with the full harmonic expansion.

All of these tutorials I added under the angular quadrature tutorials, since they are designed to showcase certain aspects directly related to the angular quadrature class.