STK_PARAM_RELIK computes the restricted likelihood of a model given data

 CALL: C = stk_param_relik (MODEL, XI, ZI)

   computes the value C of the opposite of the restricted likelihood criterion
   for the MODEL given the data (XI, ZI).

 CALL: [C, COVPARAM_DIFF, LNV_DIFF] = stk_param_relik (MODEL, XI, ZI)

   also returns the gradient COVPARAM_DIFF of C with respect to the parameters
   of the covariance function, and its derivative LNV_DIFF of C with respect to
   the logarithm of the noise variance.

 EXAMPLE: see paramestim/stk_param_estim.m