tests and all header references assume repo is cloned into mpi3

If you git clone it the default it ends up with is boost_mpi3. I don't think you should assume the name of the directory that is repo root or anything about the structure above. I should be able to add your library root to my include dirs and relative includes should work. I should be able to check out more than one copy and switch without renaming things or nesting directories.

Since many of the header names are very generic they certainly should have a leading subdirectory. But within the library I don't think navigating out of the project and back in is a good answer. I'd suggest includes in a directory called mpi3.