[jan-janssen]

This is basically reverting the changes in:
a48a902

The goal is to add support for external Executors.

[srmnitc]

@jan-janssen as we discussed, I never removed the pylammpsmpi and library interface support. Keyword execution_mode: library should be enough.

[jan-janssen]

@srmnitc It is basically the same change we did in #234 . Here is an example for using pylammpsmpi with external Executor from executorlib:
https://github.com/pyiron-dev/pylammpsmpi-with-executorlib/blob/main/example_cluster.ipynb

with FluxClusterExecutor( 
    max_workers=1, 
    block_allocation=True, 
    init_function=init_function,
    resource_dict={
        # "submission_template": submission_template, 
        # "run_time_max": 180,  # in seconds  
        # "partition": "s.cmfe",
        "cores": 2,
        "threads_per_core": 1,
    },
    pmi_mode="pmix",
) as exe:
    lmp = LammpsASELibrary(executor=exe, cores=2)

Then I want to provide this pylammpsmpi LAMMPS instance as an input to calphy.