Add cmorizer for COBE-SST2 dataset.

[bjoernbroetz]

Tasks

• Create an issue to discuss what you are going to do, if you haven't done so already (and add the link at the bottom)
• Give this pull request a descriptive title that can be used as a one line summary in a changelog
• Make sure your code is composed of functions of no more than 50 lines and uses meaningful names for variables
• Circle/CI tests pass. Status can be seen below your pull request. If the tests are failing, click the link to find out why.
• Preferably Codacy code quality checks pass, however a few remaining hard to solve Codacy issues are still acceptable. Status can be seen below your pull request. If there is an error, click the link to find out why. If you suspect Codacy may be wrong, please ask by commenting.
• Please use yamllint to check that your YAML files do not contain mistakes

New data reformatting script

• Test the CMORized data using recipes/example/recipe_check_obs.yml, to make sure the CMOR checks pass without errors
• Add the new dataset to the table in the documentation
• Tag @remi-kazeroni in this pull request, so that the new dataset can be added to the OBS data pool at DKRZ and synchronized with CEDA-Jasmin

---

Partly closes #1802

[bjoernbroetz]

@soufianekar Coud you provide this information?

[soufianekar]

ftp://ftp.cdc.noaa.gov/Datasets/COBE2/sst.mon.mean.nc