Explain the xlarge GYSELAX test case

GYSELAX/testcase_xlarge/README.md

 ## XLARGE TEST TKE
 The xlarge TKE case test corresponds to a strong scaling study performed on irene AMD. 3 simulations are performed.  
-Case0 on 93 nodes  
-Case1 on 186 nodes  
-Case2 on 372 nodes
+Case0 on 96 nodes:
+ - 768 MPI tasks
+ - 8 MPI tasks per node  
+ - 4.10 Go per MPI task
+Case1 on 192 nodes:
+ - 1536 MPI tasks
+ - 8 MPI per node  
+ - 2.25 Go per MPI task
+Case2 on 384 nodes:
+ - 3072 MPI tasks
+ - 8 MPI tasks per node
+ - 1.64 Go per MPI task
+
+The number of MPI tasks per node can be changed. To do this, you need to change the number of thread.  
+If the number of thread decreases, the number of MPI tasks per node increases. The number of MPI tasks and the number of nodes is directly computed in your subgys from the number of threads. Please, read the section **Add a new machine** in ../README.md.