Added parallel kruskal algorithm

[czgdp1807]

References to other Issues or PRs or Relevant literature

#137

Brief description of what is fixed or changed

Following changes have been made,

• Adding parallel kruskal algorithm
• Allowed custom comparator in merge_sort_parallel
• Allowed, len(array_object)

Other comments

[codecov]

Codecov Report

Merging #184 into master will increase coverage by 0.034%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master      #184       +/-   ##
=============================================
+ Coverage   96.829%   96.864%   +0.034%     
=============================================
  Files           24        24               
  Lines         1798      1818       +20     
=============================================
+ Hits          1741      1761       +20     
  Misses          57        57               

Impacted Files	Coverage Δ
pydatastructs/graphs/__init__.py	100.000% <ø> (ø)
pydatastructs/trees/binary_trees.py	96.951% <ø> (ø)
pydatastructs/graphs/algorithms.py	100.000% <100.000%> (ø)
pydatastructs/linear_data_structures/algorithms.py	100.000% <100.000%> (ø)
pydatastructs/linear_data_structures/arrays.py	100.000% <100.000%> (ø)