Fixing how visits are counted

[mnskim]

Previously visits were only updated when calling backpropagation with a reward value,
@ #48 attempted to fix by always propagating values up independently from reward.

However this introduces a new bug of overcounting visits, because every time _backpropagate is called, all ancestor nodes get .visits +1.

I think it might be better to separate visit update from _backpropagate, there could be several options:

• Visits could be updated when a node is selected, and the incrementing could be implemented in the BaseSelector class,
• Alternatively, it could be updated in search_tree.py's _select function