A function to compute the precision matrix of a GMRF on an M x N toral grid with neighbourhood size ns. Note that the precision matrix is block circulant. The returned function operates on a parameter vector as in Rue and Held (2005) pp 187.
paramprec(ns, M, N)neighbourhood size
number of cells in x direction
number of cells in y direction
a function that returns the precision matrix given a parameter vector.