Adding skiplist

[Arvind-raj06]

Adding SkipList

References to other Issues or PRs or Relevant literature

"Fixes #149". See https://github.com/codezonediitj/pydatastructs/issues/149

Brief description of what is fixed or changed

The skiplist data structure is added in the linear data structure and the number of layer is dynamic based on the probability.

Other comments

This set of code is the best I have ever seen for skiplist, yes this is also counted under SWoC. If you have any queries ping me.

[codecov[bot]]

Codecov Report

Merging #328 (5929621) into master (250ed6f) will decrease coverage by 0.006%. The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master      #328       +/-   ##
=============================================
- Coverage   98.557%   98.550%   -0.007%     
=============================================
  Files           25        25               
  Lines         3119      3243      +124     
=============================================
+ Hits          3074      3196      +122     
- Misses          45        47        +2     

Impacted Files	Coverage Δ
pydatastructs/linear_data_structures/__init__.py	100.000% <ø> (ø)
pydatastructs/utils/__init__.py	100.000% <ø> (ø)
...datastructs/linear_data_structures/linked_lists.py	99.173% <100.000%> (-0.424%)	:arrow_down:
pydatastructs/utils/misc_util.py	98.830% <100.000%> (+0.064%)	:arrow_up:

[Arvind-raj06]

@czgdp1807 can you pls take a look in that area for coverage I have added the test for the vars method too but I'm getting it as missed https://github.com/codezonediitj/pydatastructs/pull/328/files/b2e084325239a7f9d4ecb69785d82185ae94f1a7#diff-ce49dc37653769c592b712516c715500230e11637657f4039f28e179835d6fa4

[Arvind-raj06]

Coverage Done