Feature/contains

[Geson-anko]

Implemented the __contains__ method in PathDict to check if the path is contained. from issue

from path_dict import PathDIct
pd = PathDict()
pd["a","b"] = 1
print(["a"] in pd) # True
print(["a","b"] in pd) # True
print(["a","c"] in pd) # False

[Geson-anko]

@mkrd Thank you for reviewing my code! I fixed my code to "early return". Let me know if you need anything else.