Add reference implementation for parallel_phase feature #3450

Job	Run time
codespell	16s
examples_clang-format	5s
copyright_check	5s
ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	20m 22s
python_module_test_ubuntu_latest	31s
ubuntu-20.04_g++_cxx17_release_preview=ON	17m 23s
ubuntu-20.04_g++-10_cxx20_debug_preview=ON	14m 50s
examples_ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	3m 39s
ubuntu-22.04_g++-11_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	17m 48s
examples_macos-15_clang++_cxx14_relwithdebinfo_preview=ON	2m 41s
examples_ubuntu-20.04_g++_cxx17_release_preview=ON	3m 45s
examples_ubuntu-20.04_g++-10_cxx20_debug_preview=ON	4m 39s
macos-14_clang++_cxx14_relwithdebinfo_preview=ON	13m 37s
examples_windows_cl2019_cxx14_relwithdebinfo_preview=ON	6m 0s
macos-13_clang++_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	19m 28s
examples_windows_cl2022_cxx17_relwithdebinfo_preview=OFF	5m 40s
windows_cl2019_cxx14_relwithdebinfo_preview=ON	29m 17s
windows_cl2019_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	21m 19s
windows_cl2022_cxx17_relwithdebinfo_preview=OFF	15m 17s
documentation	52s
pages	0s
	3h 17m 34s

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