Add mapping dict as graph attribute in condensation.

I've found myself several times needing a mapping of original nodes to condensation nodes (ie strongly connected components). It is easy to compute that myself (especially since `condensation` function accepts precomputed SCC as a parameter). But then the function itself recalculates the mapping for building the condensation graph. Adding the mapping as graph attribute is simple, backwards compatible, and avoids redundant computations.

I've also updated `condensation` docstrings and added a test.