Add reference implementation for parallel_phase feature #3542

Job	Run time
codespell	16s
copyright_check	6s
examples_clang-format	9s
examples_ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	3m 50s
examples_ubuntu-20.04_g++_cxx17_release_preview=ON	3m 55s
examples_ubuntu-20.04_g++-10_cxx20_debug_preview=ON	4m 34s
ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	20m 6s
examples_macos-15_clang++_cxx14_relwithdebinfo_preview=ON	2m 39s
ubuntu-20.04_g++_cxx17_release_preview=ON	17m 16s
ubuntu-20.04_g++-10_cxx20_debug_preview=ON	14m 18s
macos-14_clang++_cxx14_relwithdebinfo_preview=ON	10m 39s
ubuntu-22.04_g++-11_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	17m 7s
macos-13_clang++_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	14m 18s
python_module_test_ubuntu_latest	34s
windows_cl2019_cxx14_relwithdebinfo_preview=ON	15m 2s
windows_cl2019_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	20m 40s
windows_cl2022_cxx17_relwithdebinfo_preview=OFF	15m 38s
examples_windows_cl2019_cxx14_relwithdebinfo_preview=ON	5m 59s
examples_windows_cl2022_cxx17_relwithdebinfo_preview=OFF	5m 45s
documentation	51s
pages	0s
	2h 53m 42s

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