This speeds up processings on the same dataset significantly by creating
a .level file that saves the level in which a node was contracted.
It removes the need for expensive recomputation of priorities and such.
Use with care! Running `osrm-extract` again will invalidate .level
files.
This improves preprocessing times in favour of worse query performance.
Core size can be set over the --core parameater, default is the old
behaviour to fully contract the graph.
