Use rayon-cond to use threads conditionally without manual code duplication.

[jlapeyre]

See this comment:

#428 (comment)

Sometimes one wants to choose whether to use multiple threads in a loop based on some runtime criteria. rayon-cond does the code duplication with a macro hidden behind a nice interface.

@mtreinish pointed to these sections that could be cleaned up this way.

https://github.com/Qiskit/retworkx/blob/main/src/shortest_path/floyd_warshall.rs#L189-L216
https://github.com/Qiskit/retworkx/blob/main/src/shortest_path/floyd_warshall.rs#L117-L140
https://github.com/Qiskit/retworkx/blob/main/src/shortest_path/average_length.rs#L65-L72
https://github.com/Qiskit/retworkx/blob/main/src/shortest_path/distance_matrix.rs#L73-L84

[jlapeyre]

Unfortunately, rayon's par_iter iterates over items in a collection, but rayon-cond's CondIterator iterates over references to items. So code has to be changed a bit when modifying to use rayon-cond.

[jlapeyre]

Finished examples of making this work are here #815 and reference therein.
It looks like at least some of the items in the opening comment above are still relevant.