links to existing CMOR files should be relative, not absolute

When moving the results of EarthDiagnostics to a different folder the links to other files (e.g. CMOR files from auto-ecearth POST job) are invalid.

This could be fixed by using relative symlinks, instead of absolute symnlinks.

e.g.

etourign@bscearth346:/esarchive/exp/ecearth/t00t/monthly_mean/tas_f6h> ls -l
total 0
lrwxrwxrwx 1 pechevar Earth 145 Apr 24 09:38 tas_Amon_EC-EARTH3_t00t_S19900101_r1i1p1_199001-199001.nc -> /esarchive/exp/ecearth/t00t/cmorfiles/BSC/EC-EARTH3/t00t/S19900101/mon/atmos/tas/r1i1p1/tas_Amon_EC-EARTH3_t00t_S19900101_r1i1p1_199001-199001.nc
lrwxrwxrwx 1 pechevar Earth 145 Apr 24 09:38 tas_Amon_EC-EARTH3_t00t_S19900101_r1i1p1_199002-199002.nc -> /esarchive/exp/ecearth/t00t/cmorfiles/BSC/EC-EARTH3/t00t/S19900101/mon/atmos/tas/r1i1p1/tas_Amon_EC-EARTH3_t00t_S19900101_r1i1p1_199002-199002.nc