Expand feature selection API to return population of feature sets

I've added select_feature_sets and
{moses,incremental,smd,simple}_select_feature_sets to return
feature_set_pop instead of feature_set.

feature_set_pop is just a multimap from score to feature_set.

select_features still exists (and is still used by feature-selection)
and wraps select_feature_sets.
Add --fs-demes option and implement the feature_selector part

This option (not completed yet) allows spawn several demes out of the
same exemplar, each deme using a different set of selected feature
sets.
Move metapopulation code under moses/metapopulation

Split metapopulation.{h,cc} into metapopulation.{h,cc},
metapop_params.{h,cc} and deme_expander.{h,cc}
Add multi-thread support to simple feature selection algorithm
Fix simple (keep exactly same multimap so that the unit test pass)
Clean feature selector call code (move piece of codes in methods)
More refactoring of feature selection with moses (code looks more manageable)