Merge branch 'extract_lev' into 'master'

Extract level

I just put this script to extract the levels from the 3D file coming from atmosphere diagnotic because @obellprat needed it. @obellprat if you use it and everything is fine could you confirm it here?
I don't know how this does feet with the future strategy of using CMOR. In CMOR how will it be be? all the level together or one file per level? In any case it can be usefull in the mean time.

See merge request !11