Add Climate projections CMIP6 layer

[elongano]

https://wcrp-cmip.org/cmip-phase-6-cmip6/

References:
Eyring, V., Bony, S., Meehl, G. A., Senior, C. A., Stevens, B., Stouffer, R. J., and Taylor, K. E.: Overview of the Coupled Model Intercomparison Project Phase 6 (CMIP6) experimental design and organization, Geosci. Model Dev., 9, 1937-1958, doi:10.5194/gmd-9-1937-2016, 2016.
Detailed and up-to-date description of the CMIP6 experiments protocol: https://search.es-doc.org/?project=cmip6&