Fix erroneous use of grid_latitude and grid_longitude and cleaned ocean grid fixes

[schlunma]

This PR

• renames grid_latitude and grid_longitude to cell index along first/second dimension (see Erroneous use of grid_latitude and grid_longitude #1067).
• removes duplicated code from ocean grid fixes and corresponding tests.
• adds missing tests for tos and siconc of FGOALS-g3.

I checked all affected preprocessed files (raw and regridded to a regular grid) before and after this PR with cdo diff and found no differences.

I didn't find any appearance of grid_latitude or grid_longitude in ESMValTool, so this shouldn't affect any diagnostic.

Closes #1067.

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[zklaus]

Really love the approach of unifying all these fixes! 👍 I have added some suggestions for the actual fix. Let me know what you think.