Only applies if CasidaMomentumTransfer is nonzero. Directionally averaged dynamic structure factor is calculated by averaging over the results from a set of $\vec{q}$-vectors. The vectors are generated using Gauss-Legendre quadrature scheme [see e.g. K. Atkinson, J. Austral. Math. Soc. 23, 332 (1982)], and this variable determines the order of the scheme.