add XIOS3 compatibility in 4.2
This is a copy of what was done in #170 (closed) but in the 4.2
We are not supposed to add a new feature in 4.2 but we decided that this add was very small and quite useful...
Note that, contrary to what was done in 7ab53708, we are not using the cpp syntaxe:
#ifndef key_xios3
but we used
#if ! defined key_xios3
which is more in line with was is existing in NEMO and did not require further changes such as df8b4eab