Error in  SO mask latitude values

The wrong latitude values are set in the mask used for SO SST in [gen_tmask](https://github.com/JMMP-Group/MARINE_VAL/blob/main/SCRIPT/gen_tmasks.py#L69).  It should be SO SST limits:  -70.000  -45.000  to match the observations set in [SO_sst_mean_obs.txt](https://github.com/JMMP-Group/MARINE_VAL/blob/main/OBS/SO_sst_mean_obs.txt)
  