Kalman filter log-likelihood
[llik]=filter_like(param,func,y,x,F,z)
param = a vector of parameters (sqrt of variances)
func = the function which transforms the parameters into the matrix of variances (Q and R)
y = (nx1) data vector
x = (nxk) data matrix
F = transition matrix
z = (nxl) data matrix
llik = - 2*log-likehood (+n*log(2*%pi))
// Filter_like is used by the function kalman: oresult = maxlik(filter_like,grocer_param,grocer_func,grocer_y,grocer_x,grocer_F, varargin(:))