Make some SMPI functions public

Make some SMPI functions public

When doing mixed S4U/SMPI simulations, users might want to initialize
SMPI manually, and to call functions that were private prior to this
patch. In particular, the bench functions (begin/end) can be useful.

Additionally, the callback `smpi_comm_copy_buffer_callback` must be made
public to allow S4U users to initialize the "default comm data copy
callback" in the engine manually