out-of-memory warnings/solutions

If the user asks for many GCMs/runs/periods/scenarios/years across many (many) locations, they can easily run out of memory to extract cliamte values and the large `data.table` of climate values to be downscaled.

I've seen this happen when asking for all GCMs/scenarios/periods x 3 runs for 2 700 000 point coordinates, using a 32Gb machine. The error is of the type `std::bac_alloc` which can easily be made more intuitive for the user with some messaging.

We could also foresee having `climr` actually _deal_ with this problem by, e.g.:
1. extracting/downscaling by subsets of points or combinations of climate model parameters
2. saving each subset to a csv file with `write.csv(..., append = TRUE)`

See https://stackoverflow.com/questions/78170318/error-stdbad-alloc-using-terraextract-on-large-stack-and-many-points -- extracting only the unique raster locations is not a good solution because 1) results are different and 2) the user will still run out of memory when expanding back to the full set of points.