add option for future projections

[AMBarbosa]

• decide whether this should be a separate function or some arguments added to map_risk()
• add required arguments / user inputs model, ssp, time
• document the new arguments

[dario-ssm]

Thanks Marcia! From my point of view, I think it would be more easily accesible if it is an argument. For new argument, CMIP5 data from worldclim is easy with geodata but CMIP6 worldclim CMIP6 should be used. We'll see in another moment.

[dario-ssm]

Now CMIP6 is available in geodata::cmip6_tile() or geodata::cmip6_world().

I already have a code to average all models giving the average of all models and the standard deviation that might be used to propagate predictor uncertainty. We might incorporate it in the near future.