changing version, adding author
corrections in the comments
adding checking for NA values, simplifying obtaining model covariances, reindenting the code
adding NEWS file
changing POSITIVE_DEFINITE_ERROR to INVALID_COVARIANCE_ERROR, changing zeros and NANs handling in energy_funciton.c
changing 'ZERO_EPSILON' to '1.0e-32'
checking 'eigenvalues' type parameter