Returns gauss-legendre frequency grid (kv of length Npart*order) and weights (wv) for Gramians integration.

The integration grid is divided into Npart partitions, and in each of them integration is performed using a Gauss-Legendre quadrature of order order.

Note: integration points are never located at k0 or kend, hence there is no need for special treatment as in (for e.g.) a uniform grid case (see get_unif_weights)