Made localsearch.Problem implement Comparator<Solution>.
Added container.WeightedTree#getIndexed().
Split localsearch.GeneticPopulation#peekRandom() into peekRandom() and peekWeighted().
Reworked container.WeightedTree#iterator(), implementing remove().
Added localsearch.Population#retainBest().
Reworked localsearch.GeneticLS to incorporate new Selector interface.
Implemented localsearch.TournamentSelector and RouletteWheelSelector.