Add Merge Sort Algorithm

[youknowqyh]

References to other Issues or PRs or Relevant literature

Brief description of what is fixed or changed

A natural merge sort is similar to a bottom-up merge sort except that any naturally occurring runs (sorted sequences) in the input are exploited. Example:

Start : 3 4 2 1 7 5 8 9 0 6 Select runs : (3 4)(2)(1 7)(5 8 9)(0 6) Merge : (2 3 4)(1 5 7 8 9)(0 6) Merge : (1 2 3 4 5 7 8 9)(0 6) Merge : (0 1 2 3 4 5 6 7 8 9)

Other comments

REF LINK - https://en.wikipedia.org/wiki/Merge_sort

[codecov[bot]]

Codecov Report

Merging #461 (4ac683e) into master (c0d2786) will increase coverage by 0.005%. The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master      #461       +/-   ##
=============================================
+ Coverage   98.549%   98.554%   +0.005%     
=============================================
  Files           29        29               
  Lines         3792      3806       +14     
=============================================
+ Hits          3737      3751       +14     
  Misses          55        55               

Impacted Files	Coverage Δ
pydatastructs/linear_data_structures/__init__.py	100.000% <ø> (ø)
pydatastructs/linear_data_structures/algorithms.py	99.773% <100.000%> (+0.007%)	:arrow_up:

[czgdp1807]

We already have merge_sort_parallel. Using one thread there will be equivalent to normal merge sort algorithm. Thanks for the PR though. Closing it now.