Add clique percolation (`node_x_percolation()`)

[jhollway]

Clique percolation (Palla, Derényi, Farkas & Vicsek 2005) finds overlapping
communities: a node can belong to several at once, or to none.

Reference: Nature 435:814-818, https://doi.org/10.1038/nature03607

Why this fits without a new class

node_x_clique() in R/motif_cliques.R already solved this problem. Its
documentation says why (lines 11-14):

Cliques are the strictest notion of a cohesive subgroup, and unlike the
communities returned by node_in_*() functions they overlap: a node may
belong to many cliques at once, or to none. That is why this returns an
incidence table rather than a membership vector.

So clique percolation belongs in the motif family, as
node_x_percolation(), returning a node x community incidence matrix via
make_node_motif(). No class change is needed, and no _pkgdown.yml edit
either, since the reference index already picks up contains("_x_").

Algorithm

node_x_percolation <- function(.data, clique_size = 3)

1. igraph::max_cliques(graph, min = clique_size).
2. Build the clique-overlap graph: two cliques are adjacent when they share
   clique_size - 1 nodes. Get the overlaps as tcrossprod() of the clique
   incidence matrix.
3. igraph::components() of that graph. Each component is one community.
4. A node belongs to every community holding a clique it is in. Build the node
   x community 0/1 matrix and pass it to make_node_motif().

Reuse

The preparation block at R/motif_cliques.R:46-65 should be factored into a
private helper shared with node_x_clique() rather than copied. It covers the
expect_nodes() coercion, the to_unsigned(keep = "positive") step on a signed
network, and the two-mode trick of connecting nodes that share a partner so that
a biclique becomes an ordinary clique.

Also follow node_x_clique() in calling snet_info() when nothing is found
(R/motif_cliques.R:80).

Test that matters

On a fixture with a known overlap, at least one row of the returned matrix must
sum to more than 1. Without that assertion the test suite would pass on an
implementation that quietly returns a partition.