STK_PARAM_INIT_LNV provides a rough estimate of the variance of the noise

 CALL: LNV = stk_param_init_lnv (MODEL, XI, YI)

   returns a rough estimate of the log of the noise variance computed using
   the given MODEL and data (XI, YI), using the restricted maximum likelihood
   (ReML) method. It selects the maximizer of the ReML criterion out of a
   list of possible values.

 NOTE: assumption on the model

   The model is assumed to be a stationary Gaussian process, with
   model.param(1) corresponding to the log of the Gaussian process variance.
   This assumption is currently fulfilled by all the models shipped with STK.

 See also: stk_param_estim, stk_param_init