Added AVL Tree

[czgdp1807]

References to other Issues or PRs or Relevant literature

[1] https://courses.cs.washington.edu/courses/cse373/06sp/handouts/lecture12.pdf

Brief description of what is fixed or changed

PyDataStructs now supports AVL Trees.

Other comments

This PR aims at adding AVL tree. Currently its a work in progress.

[codecov]

Codecov Report

Merging #23 into master will increase coverage by 0.303%.
The diff coverage is 96.734%.

@@              Coverage Diff              @@
##            master       #23       +/-   ##
=============================================
+ Coverage   97.761%   98.064%   +0.303%     
=============================================
  Files           15        15               
  Lines          536       775      +239     
=============================================
+ Hits           524       760      +236     
- Misses          12        15        +3

Impacted Files	Coverage Δ
pydatastructs/trees/__init__.py	100% <ø> (ø)	⬆️
pydatastructs/utils/misc_util.py	100% <100%> (ø)	⬆️
pydatastructs/trees/tests/test_binary_trees.py	100% <100%> (ø)	⬆️
pydatastructs/trees/binary_trees.py	97.553% <94.904%> (+0.427%)	⬆️