[documentation] Compare available executors

[jan-janssen]

Compare different kinds of executors to unify the interface:

Name	Package	Scaling	Interface	Comment
ThreadPoolExecutor	standard library	single node	Python
ProcessPoolExecutor	standard library	single node	Python	fast on a single node
MPIPoolExecutor	mpi4py	MPI	Python
FluxExecutor	flux	Flux	Shell
Executor	pysqa	SLURM	Python	hdf5 support
PyFluxExecutor	pympipool	Flux	Python
PyMPIExecutor	pympipool	MPI	Python
PySlurmExecutor	pympipool	SLURM	Python
SubprocessExecutor	pympipool	Subprocess	Shell	conda support

Open Questions:

• Is there any SSH support for existing Executors? How to handle transfer of files on remote locations?
• Can shell tasks and python tasks be combined in a single submission? With flux this should not be an issue and similarly with slurm.

[jan-janssen]

By now all these executors are unified and available via from executorlib import Executor. The different capabilities are now discussed as part of the different modes:

• Local Mode for running on a workstation computer.
• HPC Submission Mode for submitting individual Python functions to the job scheduler of an HPC.
• HPC Allocation Mode for submitting Python functions within the allocation of a given HPC job scheduler.