This brings coverage to 100% in all modules of the components package.

Following #1501 work, now strongly_connected_components and friends
yield sets of nodes. Added examples to the documentation for getting a
sorted list of components, and also the largest. Improved tests and
raised coverage to 100%.

Fixed a docstring example for `simple_cycles` that was relaying on
the output order of SCC.

weakly_connected_components returns now a generator of sets. Also
refactored the BFS computation which was using a single source
shortest path algorithm, as in connected_components.

Make connected_components a generator of sets of nodes, it was a generator
of list of nodes. See #1501 for discussion.

Use a plain BFS to compute them instead of using the single source
shortest path function. The latter also does a BFS but keeps track of the
distances. This is only slightly faster for small graphs (arround 5% in my
tests), but for larger graphs the speed up is more relevant (around 1.8x
faster). Also I think it's conceptually better. As @ysitu said, we have a
tendency to abuse shortest paths functions for reachability.

Also improved biconnected components examples, docstrings, and code.