Implement is_articulation to check if vertex is articulation point

[thchr]

This factors out the DFS step from articulation into a separate function articulation_dfs! and then uses this new function to implement is_articulation(g, v) which checks whether v is an articulation point of g without necessarily computing all the articulation points.

The branching on isnothing(is_articulation_pts) in articulation_dfs! is a bit inelegant - as is the fact that articulation_dfs! doesn't mutate if is_articulation_pts === nothing, but it seemed the simplest way to avoid code duplication.

Aside from these additions, I revised the doc string of articulation(g) a bit (e.g., it 1. previously misleadingly stated that g had to be connected; it does not; and 2. it changed nomenclature from "articulation point" to "cut vertex" mid-sentence).

[thchr]

Timing-wise, is_articulation(g, v) appears to give a speed-up of about a factor of ~2 in cases I looked at, compared to v ∈ articulation(g):

julia> g = path_graph(5)
julia> articulation(g) # [2, 3, 4]
julia> @btime 2 ∈ articulation($g)   # 209.656 ns (8 allocations: 704 bytes)
julia> @btime is_articulation($g, 2) # 122.850 ns (4 allocations: 464 bytes)

julia> g = path_graph(15)
julia> articulation(g) # [2:14...]
julia> @btime 9 ∈ articulation($g)   # 453.853 ns (10 allocations: 2.16 KiB)
julia> @btime is_articulation($g, 9) # 245.286 ns (4 allocations: 624 bytes)

[codecov]

Codecov Report

Attention: Patch coverage is 98.21429% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.30%. Comparing base (56e5604) to head (669093e).
Report is 4 commits behind head on master.

Files	Patch %	Lines
src/biconnectivity/articulation.jl	98.21%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #387      +/-   ##
==========================================
- Coverage   97.31%   97.30%   -0.02%     
==========================================
  Files         120      120              
  Lines        6954     6965      +11     
==========================================
+ Hits         6767     6777      +10     
- Misses        187      188       +1     

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[thchr]

Bump.