Add option to return mapped before and after n-quad indices

[dlongley]

When processing the input quads (in order) at the end of the URDNA2015 algorithm -- where N-Quads are created with the canonized blank node labels, add a property to each N-Quad string that keeps track of the original quad index. Then, after sorting the N-Quads produce a mapping of the old input quad indexes to the new sorted N-Quad indexes. Return this mapping from the algorithm as an additional output.

The output is expected to be useful in selective disclosure algorithms.