Add reference implementation for parallel_phase feature #3388

Job	Run time
codespell	12s
examples_clang-format	8s
copyright_check	5s
examples_ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	3m 36s
examples_ubuntu-20.04_g++_cxx17_release_preview=ON	3m 49s
examples_ubuntu-20.04_g++-10_cxx20_debug_preview=ON	4m 42s
ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	18m 55s
ubuntu-20.04_g++_cxx17_release_preview=ON	17m 15s
examples_macos-12_clang++_cxx14_relwithdebinfo_preview=ON	4m 0s
ubuntu-20.04_g++-10_cxx20_debug_preview=ON	15m 17s
macos-12_clang++_cxx14_relwithdebinfo_preview=ON	5m 20s
ubuntu-22.04_g++-11_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	16m 46s
macos-13_clang++_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	21m 8s
python_module_test_ubuntu_latest	32s
windows_cl2019_cxx14_relwithdebinfo_preview=ON	14m 4s
windows_cl2019_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	20m 33s
windows_cl2022_cxx17_relwithdebinfo_preview=OFF	14m 35s
examples_windows_cl2019_cxx14_relwithdebinfo_preview=ON	5m 14s
examples_windows_cl2022_cxx17_relwithdebinfo_preview=OFF	5m 57s
documentation	0s
pages	0s
	2h 52m 8s

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