Improve stdouterr_redirected file naming for multiprocessing cases

[akremin]

desispec.parallel.stdouterr_redirected currently writes to f"{basename}-rank{rank}", which is the desired behavior for MPI applications. For multiprocessing application @weaverba137 pointed out that it is a bit misleading to save all of the processes to a single file labeled "rank-0". Consider a different naming convention if the communicator isn't provided to the function, instead of defaulting to rank=0 and the same labeling scheme.