Use lazy option to load `RasterStack`?

[albangossard]

For the moment, we aren't using the lazy option that prevents from reading the whole netcdf file when we load climatic data. Maybe it's worth trying to use that option since the climatic data can have a time window much larger than the tspan provided by the user, and hence we only need a small portion of the data. We might have to create cache variables to avoid having to read data from disk during the simulation, in a similar spirit as what is done with longterm_temps.