Improve Ford Fulkerson Implementation (#745)

* ref(graph): refactor Ford Fulkerson implementation
- Added Rust doc comments to provide better documentation for the functions and the module
- Return residual graph for flow verification
- Renamed variables for better readability
- Use a macro to define the tests
- Add test for no feasible flow (`test_example_6`)

* chore(fix:fmt): remove trailing white spaces

* chore(fix[fmt]): fix no newline at the end of file

* ref: refactor ford fulkerson
- Add custom error types
- Add some edge tests

* chore: fix typo in document

* ref: add error variant of graph is not square matrix

* ref: refactor implementation
- Move error checking into a validation function
- Remove redundant comments explained variants

* chore: change `isize` to `usize` in graph reprentation

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Co-authored-by: Piotr Idzik <65706193+vil02@users.noreply.github.com>