`alpha_centrality()` should be updated for new `as_adj()` weight handling

[szhorvat]

After #906 is fixed, alpha_centrality() should be updated to use the new weights parameter of as_adj().

In short, alpha_centrality() needs to be able to build an adjacency matrix based on an arbitrary weight vector. As of igraph 1.5, as_adjacency_matrix() cannot do this directly. Therefore alpha_centrality() copies the graph and adds a special edge attribute just to be able to transfer the weights.

[maelle]

Ah good, I was looking at it in my branch since I was getting my own deprecation messages in the tests 🤪

[maelle]

I do not understand how this case can come up:

rigraph/R/centrality.R

Lines 1537 to 1538 in 238ec98

	} else if (is.character(weights) && length(weights) == 1) {
	## name of an edge attribute, nothing to do

[maelle]

@szhorvat if you are able to explain 🙏

[maelle]

see what I made of alpha.centrality.dense for now in #1170