Rename public-facing code/variables from "visits" to "nodes"

[dubslow]

The --visits option is kept as hidden for compatibility. --visits and --nodes are equivalent.

This would remove some ambiguity in public facing engine-use documentation, including what "playouts" means.

Acceptance of this PR should be accompanied by simultaneous wiki updates to reflect the change.

[evalon32]

I worry a little about advertising equivalence between visits and nodes given that they are not really equivalent: a terminal node can be visited many times without additional expansion, resulting in visits > nodes and sometimes even visits >> nodes.

[dubslow]

The wiki already has a big fat asterisk for this corner case. But the code doesn't change, and this is much more understandable I think. And besides the same thing can be said for playouts, and the difference between playouts and visits is far more confusing than the difference between nodes and visits. So not perfect, but still a significant improvement on the current situation I think.