Added tests

[HarsheetKakar]

References to other Issues or PRs or Relevant literature

Fixes #19

Brief description of what is fixed or changed

Added tests for mary node, binomial tree node and added test_m_ary_trees.py

Other comments

Added str function for mary node else it would take it as a TreeNode and raise error because there are no left and right attributes

[codecov]

Codecov Report

Merging #222 into master will increase coverage by 1.390%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master      #222       +/-   ##
=============================================
+ Coverage   97.123%   98.513%   +1.390%     
=============================================
  Files           24        24               
  Lines         2016      2018        +2     
=============================================
+ Hits          1958      1988       +30     
+ Misses          58        30       -28     

Impacted Files	Coverage Δ
pydatastructs/utils/misc_util.py	100.000% <100.000%> (+9.259%)	⬆️
pydatastructs/trees/m_ary_trees.py	90.625% <0.000%> (+56.250%)	⬆️