References for Markov random fields
    
    Besag, J.: (1974): Spatial interaction and the statistical analysis
    of latttice systems (with discussion). JRSS B 36:
    192-236.
    
    Gelfand et al., ch. 1213.
    
    Geyer, C.J. amd Thompson, E. A. (1992): Constrained Monte Carlo
    maximum likelihood for dependent data (with discussion). JRSS B 54: 657-699.
    
    Lindgren, F. and Rue, H. (2013), Bayesian
      Spatial and Spatio-temporal Modelling with R-INLA,  J.
      Stat. Softw.
    
    Rue, H. and Held, J. (2005): Gaussian
Markov
      random fields: theory and applications. Chapman &
    Hall/CRC
    
    Simpson, D., Lindgren, F., and Rue, H. (2010), In order to make
    spatial statistics computationally feasible, we need to forget about
    the covariance function, Environmetrics, 23, 6574.
    
    Whittle, P. (1963): Stochastic processes in several dimensions. Bull. ISI 40: 974-994.