Added abstract methods for LinkedList base class

[HarsheetKakar]

References to other Issues or PRs or Relevant literature

Fixes #187

Brief description of what is fixed or changed

Added abstract methods which are common in DLL and SLL.

Other comments

[codecov]

Codecov Report

Merging #223 into master will increase coverage by 0.002%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master      #223       +/-   ##
=============================================
+ Coverage   98.513%   98.516%   +0.002%     
=============================================
  Files           24        24               
  Lines         2018      2022        +4     
=============================================
+ Hits          1988      1992        +4     
  Misses          30        30               

Impacted Files	Coverage Δ
...datastructs/linear_data_structures/linked_lists.py	99.613% <100.000%> (+0.006%)	⬆️