Updated Welsh-Powell from outdated API (assuming color attributes can change the display). Added documentation

Use the BarabasiAlbertGenerator instead.

Change the way that BaseGenerator is keep nodes and edges informations. New way is using an internal graph.

Fixed raw connected component count.

In ConnectedComponents, the number of components is available either via
a counter (connectedComponents), or by using the size of the
connectedComponentSize map. The former allow easy access to the number of
components (raw count), the second to filter the result based on component
size restriction at a cost of a more complex operation.

The count update could, in some cases, create empty connected components. Those
were counted and lead to erroneous results where the raw count of connected
components was used. Empty connected components are now filtered out during the
raw and size-based count.