Put the source files in a dedicated directory

src/Makefile

src/hierarchical.cpp

src/masterworker.cpp

src/mw_addslave.cpp

src/parall_constants.h

src/parall_internal.h

src/perf.cpp

src/products.h

src/randomfilling.c

src/sequential.cpp

src/tensormatrix.cpp

src/tensormatrix.h

src/tensormatrix_mpi.cpp

src/utils.cpp

src/utils.h

src/utils_parall.cpp

src/utils_parall.h