Modified LinkedList API

[czgdp1807]

References to other Issues or PRs or Relevant literature

Brief description of what is fixed or changed

For consistency, data is made optional, and key attribute is added to LinkedListNode.

Other comments

[codecov]

Codecov Report

Merging #191 into master will increase coverage by 0.017%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master      #191       +/-   ##
=============================================
+ Coverage   96.864%   96.881%   +0.017%     
=============================================
  Files           24        24               
  Lines         1818      1828       +10     
=============================================
+ Hits          1761      1771       +10     
  Misses          57        57               

Impacted Files	Coverage Δ
...datastructs/linear_data_structures/linked_lists.py	99.607% <100.000%> (-0.393%)	⬇️
...datastructs/miscellaneous_data_structures/stack.py	100.000% <100.000%> (ø)
pydatastructs/utils/misc_util.py	90.000% <100.000%> (+1.111%)	⬆️