add vertex itself to nodes' neighborhood in PSCAN

[dawnranger]

The neighborhood of each node in PSCAN should contain itself.

The neighborhood of a vertex is defined in the paper of SCAN:

DEFINITION 1 (VERTEX STRUCTURE)
Let v ∈ V, the structure of v is defined by its neighborhood,
denoted by Γ(v)
Γ(v) = {w ∈ V | (v,w) ∈ E} ∪ {v}
In Figure 1 vertex 6 is a hub sharing neighbors with two clusters.
If we only use the number of shared neighbors, vertex 6 will be
clustered into either of the clusters or cause the two clusters to
merge. Therefore, we normalize the number of common neighbors
by the geometric mean of the two neighborhoods’ size.

If vertex itself is not contained, similarities of all node pairs will be smaller than the expected value.

For example, if two connected vertices have no common neighbors, the similarity will be 0. Which is the same as node pairs that are not connected by an edge. This is apparently not correct.