feat: add Parent Pointer Tree

[FamALouiz]

References to Issue

Resolves #325

Brief description of what is implemented

• Added parent pointer tree for python backend
• Added parent pointer tree tests

[LostSnowfluff]

In my opnion, for add a new data structure or algorithm for public, the final interception is reserved by czgdp1807.

First step is append the class or function in the beginning array __all__ of the file. Like:

__all__ = [
    'MAryTree',
    'ParentPointerTree'
]

Secondly, add the API in the module's __init__.py file. Like

from .m_ary_trees import (
    MAryTreeNode, MAryTree,
    ParentPointerTreeNode, ParentPointerTree
)

For ParentPointerTreeNode, you can check the above steps.

Three, after above two steps, you can import ParentPointerTree by

from pydatastructs import MAryTree, ParentPointerTree

Lastly, update your new datastructure in docs.

[FamALouiz]

Yea, points 1, 2, and 4 are valid. I had forgotten to add them.

As for point 3, I was following the importing style in the other files. In the other files, they important it differently. For example,

from pydatastructs.linear_data_structures.arrays import ArrayForTrees

However, The points are nice regardless thanks :)