Some fixes in GP framework

1)  Degrees of freedom (m_df) field moved from CLikelihoodModel to CStudentsTLikelihood. So now we can use Student's-t likelihood with different values of degrees of freedom.
2) A small fix in CLaplacianInferenceMethod::update_alpha() method. Now it uses cholesky instead of computing colPivHouseholderQr() for linear problem solving.