The new skeleton consistency algorithm now differs from the
orientation consistency algorithm only in the definition of consistency.
When the parameter consistent is set to skeleton:
The graph will be oriented during each iteration;
A candidate conditioning node will be considered as consistent as long
as it is on at least one path connecting both end nodes of interest,
regardless of the orientations of the edges or whether the candidate
node is within the neighborhood of the end nodes.
Fix comments in environment.h.
Conditioning nodes involving bi-directed edges are now considered
consistent again.
consistentis set toskeleton:environment.h.