Fix relative include in mpi.hpp

[cwpearson]

kokkos-comm/src/KokkosComm/mpi/mpi.hpp

Line 21 in a968a13

#include "../concepts.hpp"

[dssgabriel]

PR #122 should fix this. If we merge it, I have another PR ready that updates all the includes with the <KokkosComm/...> syntax. I preferred splitting them so not to touch every file at the same time as these are pretty big changes.

Additionally, @cedricchevalier19 mentioned that having relative path includes may improve compilation speed.

[nicoleavans]

PR #122 was merged; how best to proceed? <KokkosComm/...> syntax seems like a good solution.

[dssgabriel]

I can make a quick PR replacing all KokkosComm-related includes with the <KokkosComm/...> system-style includes (I may even have a working branch with this change at the ready).

However, in a discussion with @cedricchevalier19, he mentioned that these kinds of includes may have a negative impact on compile times (see previous message in this thread). I didn't notice any slowdowns for KokkosComm builds but we may want to take that into consideration before changing all the paths.
Any suggestions before we proceed?