Added parallel merge sort

[czgdp1807]

References to other Issues or PRs or Relevant literature

Fixes #128

Brief description of what is fixed or changed

Other comments

[codecov]

Codecov Report

Merging #177 into master will increase coverage by 0.078%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master      #177       +/-   ##
=============================================
+ Coverage   96.735%   96.813%   +0.078%     
=============================================
  Files           23        24        +1     
  Lines         1746      1789       +43     
=============================================
+ Hits          1689      1732       +43     
  Misses          57        57               

Impacted Files	Coverage Δ
pydatastructs/linear_data_structures/__init__.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)