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Add longest_path_length to rustworkx_core::dag_algo (#1605)

* Add longest_path_length to rustworkx_core::dag_algo

This commit builds off of #1577 which improved the performance of the
`longest_path` function to avoid allocating on each iteration of the
loop. It was pointed out in that PR and #1563 that storing and returning
a path for applications that only care about the length is unnecessary
overhead. To provide an option for such cases a new function is added to
offer a better alternative for these use cases.

* Propogate None for partial cmp

Since path weights might be floating point numbers or other types which
don't implment Ord we need to handle the case where the comparison isn't
possible (e.g. for floats the presence of NaN). Previously we just were
treating this as equal which is not correct. This commit updates the
logic in the path length function to just return None in the presence of
non comparable elements along the longest path and adds documentation to
tell users about this and suggestions on how to handle this case in the
weight functions.

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