Refactored graphs

[czgdp1807]

References to other Issues or PRs or Relevant literature

Brief description of what is fixed or changed

Consistency has been introduced in the internal representations of graphs.

Other comments

[codecov]

Codecov Report

Merging #225 into master will increase coverage by 1.394%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master      #225       +/-   ##
=============================================
+ Coverage   97.123%   98.517%   +1.394%     
=============================================
  Files           24        24               
  Lines         2016      2024        +8     
=============================================
+ Hits          1958      1994       +36     
+ Misses          58        30       -28     

Impacted Files	Coverage Δ
pydatastructs/graphs/adjacency_list.py	100.000% <100.000%> (ø)
pydatastructs/graphs/adjacency_matrix.py	100.000% <100.000%> (ø)
pydatastructs/utils/misc_util.py	100.000% <100.000%> (+9.259%)	⬆️
...datastructs/linear_data_structures/linked_lists.py	99.613% <0.000%> (+0.006%)	⬆️
pydatastructs/trees/m_ary_trees.py	90.625% <0.000%> (+56.250%)	⬆️