Efficiently receive data in an existing buffer

Property definitions

mpi $ MPI :: recv_into
	# Efficiently receive data in an existing buffer
	fun recv_into(buffer: Receptacle, at, count: Int, source: Rank, tag: Tag, comm: Comm)
	do
		buffer.recv(self, at, count, source, tag, comm)
	end
lib/mpi/mpi.nit:72,2--76,4