Create data stub component for `tgav`

[crvernon]

Create a data stub component to directly extract tgav from an fldgen emulator output (rds).

[abigailsnyder]

• specifically tgav information for the ESM runs used to train the emulator.
• This stub will extract the actual tgav component from the emulator as well as labeling information for scenarios and years from other components of the emulator
• note that if ESM runs are all ISIMIP data, the quantity with the variable name tgav is actually land average temperature and not global average temperature as the name suggests.

[abigailsnyder]

Attached: A compiled Rmarkdown notebook doing this extraction and labeling for a trained emulator from /pic/projects/GCAM/GE/drought-expt/fldgen-emulators
extracting-tgav-from-trained-emu.zip