Change max_paths parameter name to cutoff.
Refactor approximation algorithm for k-components.

1. Use the new NetworkX interfaces to connectivity for the exact
local node connectivity computation.

2. Separate the cliques heuristic part in a new function.

3. Small code improvements mostly cosmetic: avoid 'while True' and
remove superfluous if/else blocks.

I've tested that it has the same accuracy and approx the same speed
than the old implementation.