Maintaining a list of obs data sets for each variable used in REF diagnostics

[rswamina]

This is an issue to keep track of the number of data sets available for each variable used in REF diagnostics. This is to ensure we have at least one obs dataset on Obs4MIPS for every variable.

| Variable | Diagnostic used in | Datasets used (highlight datasets already in Obs4MIPS by making them bold|)
|--------------|-----------|------------|
| tas | | |
| pr | | |

Tagging people who might be able to add to the table initially. Please add others as needed : @dhegedus99 , @nocollier , @climate-dude

[dhegedus99]

Would you like me to set up a new project on github (similar to REF reference datasets)? Or just put them in the comments here?