api: revamp custom coefficients API #7022

Job	Run time
pytest-ubuntu-py311-gcc11-noomp-base	49m 23s
pytest-ubuntu-py311-gcc11-noomp-adjoint	38m 40s
pytest-ubuntu-py312-gcc12-omp-base	1h 6m 44s
pytest-ubuntu-py312-gcc12-omp-adjoint	49m 49s
pytest-ubuntu-py39-gcc7-omp-base	52m 14s
pytest-ubuntu-py39-gcc7-omp-adjoint	39m 55s
pytest-ubuntu-py310-gcc10-noomp-base	50m 26s
pytest-ubuntu-py310-gcc10-noomp-adjoint	38m 41s
pytest-ubuntu-py312-gcc13-omp-base	1h 6m 23s
pytest-ubuntu-py312-gcc13-omp-adjoint	51m 14s
pytest-ubuntu-py39-gcc9-omp-base	53m 2s
pytest-ubuntu-py39-gcc9-omp-adjoint	39m 40s
pytest-osx-py312-clang-omp-base	46m 49s
pytest-docker-py39-gcc-omp-base	46m 21s
pytest-docker-py39-gcc-omp-adjoint	35m 27s
pytest-docker-py39-icx-omp-base	37m 50s
pytest-docker-py39-icx-omp-adjoint	35m 0s
	13h 17m 38s

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