masking tos (and maybe other variables) over land

Hi,

With @rfernand and @pabretonniere, we found an issue in the files post-processed with Earth diagnostic:

The sea surface temperature (SST) over continents is equal to 273.15 °K over the continents, whereas it should be set to missing values. Currently, we cannot differentiate the regions where the SST is effectively equal to 0°C with the continental regions. That's the case for example in /esnas/exp/ecearth/i03i/monthly_mean/tos_f3h/tos_Omon_EC-EARTH_i03i_S19841101_r5i1p1_198503-198506.nc Thanks

Martin