The number of steps needed by the solver to process self

It is just a useless generation number, but could be used to evaluate the behavior of search algorithms.

Property definitions

ai $ SearchNode :: steps
	# The number of steps needed by the solver to process `self`
	# It is just a useless generation number, but could be used to evaluate
	# the behavior of search algorithms.
	var steps: Int = 0
lib/ai/search.nit:667,2--670,19