Add/enable an alternative method (Faust::compute_n_apply_grad1()) to...

Add/enable an alternative method (Faust::compute_n_apply_grad1()) to compute-apply the factor gradient in C++ PALM4MSA 2020 impl.

The old way of computing the gradient is kept in function Faust::compute_n_apply_grad1().

This commit should almost close issue #150 but it remains an small extra work to totally reach the 2016 performances on the MEG matrix factorization.

Related updates: timers of factor gradient and spectral norms + a verbose mode to compute/display them in palm4msa2() (is_verbose argument).