Fixed AdjacencyList.add_vertex

[czgdp1807]

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[codecov]

Codecov Report

Merging #320 (e64271e) into master (592dade) will increase coverage by 0.000%.
The diff coverage is 100.000%.

@@            Coverage Diff            @@
##            master      #320   +/-   ##
=========================================
  Coverage   98.828%   98.828%           
=========================================
  Files           25        25           
  Lines         3072      3073    +1     
=========================================
+ Hits          3036      3037    +1     
  Misses          36        36           

Impacted Files	Coverage Δ
pydatastructs/graphs/adjacency_list.py	100.000% <100.000%> (ø)