Correct 2nd bug in vertmeansal (valid_min) + change format of head and tail commands to make it work with older versions

cdo can change the naming of the dimensions which prevents cdftools to work properly; this is now corrected in the ohc function in case it happens

When requesting monthly mean outputs, the daily mean outputs will also be concatenated and monthly means built from these daily means

This test is not suitable anymore since the naming convention has changed in diags to be the same as MMO

if several members, oce will exist for previous members whereas sstsssmld rather for the followings so that lstfiles can be empty

if only one chunk, cdo mergetime is not useful and it would not work because the input and output files would have the same name

oce needs to be before sstsssmld so that oce is not already in the workdir when sstsssmld is treated