Fix pipelined transfers for velocity. All cases ok (1k, 2k, MS ans SingleScale). Need comm overlap for 1k.

Fix last bugs in subsets. All tests ok except python advection and penalization (--> obstacles obsolete)

Fix last bugs in subsets. All tests ok except python advection and penalization (--> obstacles obsolete)

Review mesh mechanism, add/fix subsets. All tests ok except those with obstacles somewhere. Work in progress