[WIP] Added data structure for graphs

[czgdp1807]

References to other Issues or PRs or Relevant literature

Brief description of what is fixed or changed

PyDataStructs now support Graph

Other comments

This PR is in a draft state.

[codecov]

Codecov Report

Merging #17 into master will decrease coverage by 13.085%.
The diff coverage is 36.082%.

@@              Coverage Diff               @@
##            master       #17        +/-   ##
==============================================
- Coverage   97.761%   84.675%   -13.086%     
==============================================
  Files           15        17         +2     
  Lines          536       509        -27     
==============================================
- Hits           524       431        -93     
- Misses          12        78        +66

Impacted Files	Coverage Δ
pydatastructs/__init__.py	100% <100%> (ø)	⬆️
pydatastructs/trees/space_partitioning_trees.py	96.825% <100%> (-0.794%)	⬇️
pydatastructs/graphs/__init__.py	100% <100%> (ø)
pydatastructs/graphs/graphs.py	31.868% <31.868%> (ø)
pydatastructs/trees/binary_trees.py	93.406% <0%> (-3.72%)	⬇️
...datastructs/miscellaneous_data_structures/stack.py	88.636% <0%> (-3.672%)	⬇️
pydatastructs/linear_data_structures/arrays.py	97.56% <0%> (-2.44%)	⬇️
.../miscellaneous_data_structures/tests/test_stack.py	100% <0%> (ø)	⬆️
pydatastructs/trees/tests/test_binary_trees.py	100% <0%> (ø)	⬆️
... and 4 more

[czgdp1807]

This needs a reconsideration. I will continue my work once GraphNode is added.