feat(sl): Add graph traversal utilities

• Add the GraphTraverser base class which isdesigned to simplify the implementation of utilities that traverse a TOSA graph, performing some action on reaching each operator, entering/exiting blocks, etc.
• Add an example traverser GraphPrinter and an executable graph_printer which traverses a graph printing its structure in a more readable way.
• Include some documentation in the form of a README.

Signed-off-by: Ian Tayler Lessa ian.taylerlessa@arm.com