Aggregate SIM2 daily data for each study area polygons

The get_sim2_daily function returns all SAFRAN grids that intersect the provided sf/bbox/coords.

For studies, it needs to be aggregated by polygons (see get_soil_depth and get_soil_awc).

So, need to improve get_sim2_daily function by:

• adding a new argument aggregate for each method, and updating the code
• or create a new function that calls get_sim2_daily and processes the returned object
• etc.