Add support to build diy::mpi as a library

CMakeLists.txt

cmake/DIYConfigureMPI.cmake

cmake/mpi_types.h

examples/io/test-io.cpp

examples/serialization/CMakeLists.txt

include/diy/assigner.hpp

include/diy/detail/master/communication.hpp

include/diy/io/bov.hpp

include/diy/master.hpp

include/diy/mpi.hpp

include/diy/mpi/collectives.cpp

include/diy/mpi/collectives.hpp

include/diy/mpi/communicator.cpp

include/diy/mpi/communicator.hpp

include/diy/mpi/config.hpp

include/diy/mpi/constants.hpp

include/diy/mpi/datatypes.cpp

include/diy/mpi/datatypes.hpp

include/diy/mpi/diy-mpi-export.h

include/diy/mpi/environment.cpp