[feature] Serialise executors

[jan-janssen]

As the Executors in pympipool typically use zmq to connect the executor to the process which is executing the functions it should in principle be possible to serialise the executor - store the information of the executor - and then deserialise the same object again. Still it is not clear what happens to the messages already submitted and in addition it is also not clear if the server process is started by the executor or by the process executing the python functions.

[jan-janssen]

Currently the Executor starts the zmq and the process which is executing the tasks is just connecting to it. This has to be changed to be able to serialise the Executor object - https://github.com/pyiron/pympipool/blob/main/pympipool/backend/serial.py

[jan-janssen]

The flux --parent command allows flux to connect to a parent instance rather than creating a sub instance - https://flux-framework.readthedocs.io/projects/flux-core/en/latest/man1/flux.html#cmdoption-flux-p

[jan-janssen]

Nested flux execution was implemented in #382