Move stats.n_expansions increment after logging

That way what is logged is coherent with what the stucture stats holds
during deme management.
Add demeID in pbscored_combo_tree

In order to know from where a candidate comes from we store the
demeID (currently just the generation index) besides the combo tree
and its score. To do that the std::pair pbscored_combo_tree has been
expanded to a triplet with demeID as last element.

Note that only local_moses to supports that feature. mpi and
distributed moses store 0 and demeID. To find where to fix it search
for "@todo demeID".
Log the exemplar features as well as the new selected during feature selection within moses