[major] Refactor the Executor interface (#548)

* [major] Refactor the Executor interface

Replace the backend parameter from the initialization of the Executor class with individual executor classes for the individual backends:
* `Executor(backend="local")` becomes `from executorlib import LocalExecutor§
* `Executor(backend="flux_allocation")` becomes `from executorlib import FluxAllocationExecutor§
* `Executor(backend="flux_submission")` becomes `from executorlib import FluxSubmissionExecutor§
* `Executor(backend="slurm_allocation")` becomes "SlurmAllocationExecutor"
* `Executor(backend="slurm_submission")` becomes `from executorlib import SlurmSubmissionExecutor`

This has two advantages: On the one hand it is less error prone to mistyping the backend name, as the user can use auto completion to import the right module. On the other hand it is more consistent with the standard library which defines the `ProcessPoolExecutor` and the `ThreadPoolExecutor`, rather than a `backend` parameter.

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* fix test

* update benchmark

* move interfaces to individual modules

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* fixes

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* remove create

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* remove redundant functionality

* Move graph tests

* remove core check

* clean up

* more tests

* fix import

* more fixes

* Rename LocalExecutor to SingleNodeExecutor

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* test block allocation

* fix

* flux fix

* Rename interfaces

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* fix docstrings

* add docstrings to create_*_executor() function

* fix test

* merge

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

.github/workflows/unittest-flux-mpich.yml

@@ -34,4 +34,4 @@ jobs:
       timeout-minutes: 5
       run: >
         flux start
-        python -m unittest tests/test_flux_executor.py tests/test_executor_backend_flux.py tests/test_cache_executor_pysqa_flux.py;
+        python -m unittest tests/test_flux_executor.py tests/test_executor_backend_flux.py tests/test_cache_executor_pysqa_flux.py tests/test_plot_dependency_flux.py;

.github/workflows/unittest-flux-openmpi.yml

@@ -34,7 +34,7 @@ jobs:
       timeout-minutes: 5
       run: >
         flux start
-        coverage run -a --omit="executorlib/_version.py,tests/*" -m unittest tests/test_flux_executor.py tests/test_executor_backend_flux.py tests/test_cache_executor_pysqa_flux.py;
+        coverage run -a --omit="executorlib/_version.py,tests/*" -m unittest tests/test_flux_executor.py tests/test_executor_backend_flux.py tests/test_cache_executor_pysqa_flux.py tests/test_plot_dependency_flux.py;
         coverage xml
       env:
         PYMPIPOOL_PMIX: "pmix"