Added the ImportMode enum (for off processor importing)
Switched from MPI_Send/MPI_Recv to MPI_Isend/MPI_Irecv, which are more safe, in that the sender waits for the receiver to complete the reception before changing its buffer.
Removed the Zero import mode and added Max/Min
Modified a bit the importer
First serious complete draft of the Vector class. Now it provides all the methods needed by basic distributed computing.