Investigate needed precision for output variables

[JanStreffing]

Currently we write by default mostly single precision, and salt in double precision:

Example section from namelist.io:

           'MLD2      ',1, 'm', 4,
           'MLD3      ',1, 'm', 4,
           'tx_sur    ',1, 'm', 4,
           'ty_sur    ',1, 'm', 4,
           'temp      ',1, 'm', 4,
           'salt      ',1, 'm', 8,
           'N2        ',1, 'm', 4,

For CMIP7, data output frequencies will be high. We should investigate for which variables what precision is really needed. We need metrics about error made with reducing precision.