[WIP] Added binary tree traversals

[czgdp1807]

References to other Issues or PRs or Relevant literature

[1] https://en.wikipedia.org/wiki/Tree_traversal

Brief description of what is fixed or changed

PyDataStructs supports Binary Tree Traversals

Other comments

Currently, there is a bug in _post_order. It needs to be fixed. Moreover _out_order has to be added.

[codecov]

Codecov Report

Merging #21 into master will decrease coverage by 0.42%.
The diff coverage is 95.876%.

@@              Coverage Diff              @@
##            master       #21       +/-   ##
=============================================
- Coverage   98.095%   97.674%   -0.421%     
=============================================
  Files           15        15               
  Lines          420       516       +96     
=============================================
+ Hits           412       504       +92     
- Misses           8        12        +4

Impacted Files	Coverage Δ
pydatastructs/trees/__init__.py	100% <ø> (ø)	⬆️
pydatastructs/trees/tests/test_binary_trees.py	100% <100%> (ø)	⬆️
...datastructs/miscellaneous_data_structures/stack.py	92.307% <75%> (-3.147%)	⬇️
pydatastructs/trees/binary_trees.py	96.815% <96.969%> (+0.111%)	⬆️