Add the "auto-detect the current allocation" feature

[DilumAluthge]

Most of the time, I think that users will only be working with a single cluster scheduler, and thus they can just use the relevant ClusterManager directly.

However, in some situations, I think that it might be useful to be able to write a single script that is agnostic to the specific cluster scheduler. In those cases, it would be nice to auto-detect which cluster scheduler is active, and then automatically use the correct ClusterManager.

This PR adds an experimental non-public addprocs_autodetect_current_scheduler() function that implements this.

Note: The addprocs_autodetect_current_scheduler() function should be run from inside an active allocation. So e.g. for Slurm, you would first get an allocation (e.g. sbatch or salloc), and then run this function inside the allocation.