Add weaklaplacian(P,n) and grammatrix(P,n) for nxn truncations? n can be an Int or Block

[dlfivefifty]

I recently added jacobimatrix(P,n) to ClassicalOrthogonalPolynomials.jl as it was sort of needed for n x n truncations that preserved the Tridiagonal structure. But perhaps this interface makes sense for other operators like weaklaplacian and grammatrix?