cdo sellevel,0 selects depth 0m which works fine when the depth axis is not...

cdo sellevel,0 selects depth 0m which works fine when the depth axis is not defined; nemo does not read the depth axis so it does not matter at runtime whether the depth axis is defined or not; when building the restarts, it is safer to define the depth axis to avoid mistakes; then ncks -d z,0 needs to be used rather than cdo sellevel,0 to work on restarts for which the depth axis is defined; this commit needs to be reverted in case someone notices a case for which ncks -d z,0 does not work and another more general solution should be searched if this happens