Simplify Marginals and add docs #4621

Job	Run time
ubuntu-clang-tbb Release	27m 32s
ubuntu-clang-deprecated Release	25m 0s
ubuntu-clang-cayleymap Release	20m 55s
ubuntu-clang-system-libs Release	21m 30s
ubuntu-clang-quaternions Release	25m 22s
ubuntu-build-examples Release	25m 18s
ubuntu-no-unstable Release	20m 36s
	2h 46m 13s

Provide feedback