api: Cleanup and improve SubFunction #5749

Job	Run time
pytest-ubuntu-py311-gcc11-noomp-base	1m 12s
pytest-ubuntu-py311-gcc11-noomp-adjoint	1m 13s
pytest-ubuntu-py38-gcc12-omp-base	1m 23s
pytest-ubuntu-py38-gcc12-omp-adjoint	1m 15s
pytest-ubuntu-py37-gcc7-omp-base	1m 21s
pytest-ubuntu-py37-gcc7-omp-adjoint	1m 29s
pytest-ubuntu-py310-gcc10-noomp-base	1m 27s
pytest-ubuntu-py310-gcc10-noomp-adjoint	1m 29s
pytest-ubuntu-py38-gcc8-omp-base	1m 25s
pytest-ubuntu-py38-gcc8-omp-adjoint	1m 42s
pytest-ubuntu-py39-gcc9-omp-base	1m 12s
pytest-ubuntu-py39-gcc9-omp-adjoint	2m 0s
pytest-osx-py37-clang-omp-base	5m 8s
pytest-docker-py39-gcc-omp-base	3m 14s
pytest-docker-py39-gcc-omp-adjoint	3m 7s
pytest-docker-py39-icx-omp-base	6m 31s
pytest-docker-py39-icx-omp-adjoint	6m 21s
	41m 29s

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