DFS processing finished nodes

**Describe the bug**
If a node appears several times on the stack, then it will be processed several times in the `else` branch.
https://github.com/cheran-senthil/PyRival/blob/master/pyrival/graphs/dfs.py

**Expected behaviour**
I would expect the implementation to process each node only once after its subtree is finished.
I think it would be suitable to add something like this after line 18:
`if finished[start]: continue`