Linked lists

[rohansingh9001]

Added linked-lists.py in linear-data-structures
Contains a Doubly Linked List

References to other Issues or PRs or Relevant literature

Partially Fixes issue #49 which requests to add linked lists.
Only Doubly Linked List Added so far Single Linked List Remains

Brief description of what is fixed or changed

API for the additions are as follows -

API for Double Linked List:

1)A Doubly linked list should be initialised with `DoubleLinkedList()`

2)Elements can be pushed at the start of the list using `.appendleft(data)`

3)Elements can be appended at the end using `.appendright(data)`

4) can be popped from the list at a certain index by `.pop(index)`

5)`__getitem__` and `__setitem__` support using python syntax,
eg -
`list[1]`
`list[5] =4`

6)Print the Linked List in List form using `print()` function
eg -
if elements in linked list named `sample_list` are 1-2-3-4
`>>>print(sample_list)`
`[1,2,3,4]`

7)Return length of the Linked List using `len()` function.

8)`.popright()` and `.popleft()` pops items from the right and the left respectively.

9)`.insertAt(index, data)` inserts a new Node at the index.

10)`.insertBefore(Node, data)` and `.insertAfter(Node, data)` inserts a new Node before and after the input Node respectively.

Other comments

[rohansingh9001]

Work for Single Linked List is in progress.

[codecov]

Codecov Report

Merging #50 into master will decrease coverage by 9.248%.
The diff coverage is 17.482%.

@@              Coverage Diff              @@
##            master       #50       +/-   ##
=============================================
- Coverage   98.003%   88.755%   -9.249%     
=============================================
  Files           17        18        +1     
  Lines         1102      1245      +143     
=============================================
+ Hits          1080      1105       +25     
- Misses          22       140      +118

Impacted Files	Coverage Δ
...ructs/linear_data_structures/tests/linked_lists.py	17.482% <17.482%> (ø)