Crossover for dag and tree structures

[YamLyubov]

Now crossover does not work for trees and dag (try to launch graph_search with one point crosssover). Problems:

• stack overflow at the begining of the launch (structural_equivalent_nodes)

• cycled graph appiars during crossover (update_subtree)

Possible causes:

• incorrect graph_has_cycle function
• crossover was implemented not for classical trees and dags but for reversed ones
• two graphs while crossover contain nodes with the same uids because of the way adaptation for nx graphs is implemented