cleaning/upgrade of MPI interface

[smasson]

Context

1) Review, simplification and cleaning of the MPI interface is needed
2) There is a few bugs:

• potential bug with the corners if nn_hls > 1 (or 2?) with point-to-point communication
• collective communications is not working with BDY (as BDY has its own exchanges)

3) There is other things to further improve:

• do not send/receive if send/receive only land points (depend on nn_hls)
• send/receive only the oce values (checking mask, depend on nn_hls)
• keep in memory the send/receive buffers (if they are not too big for ex < jpi*jpj) from one call to another -> for point-to-point put the mpi_waitall at the beginning of next call
• make sure the north pole folding does not need east-west comm. done if nogather = F. to be done if nogather = T... so we could place it where we want (no need to wait for east-west comm).
• if north pole folding: force the last row of MPI-subdomains to have jpj much smaller (the smallest value?) so they go faster and other don't wait for them

[smasson]

changed to #2598...