download ECMWF SEAS51 data

[zackarno]

This PR downloads all available historical ECMWF SEAS51 precipitation forecast data over the Central American Dry Corridor (CADC).

• IMHO - there is not really much to review - perhaps just a quick look at the download script here : data-raw/ecmwf_seas51.R ? Think the exploration folder can be ignored in this PR
• The reason I don't think it needs much review is because we are awaiting feedback from partners on which forecasts will be recommended (they are discussing national forecasts). Therefore the purpose of this branch was just to set up the template for scripts to download the ECMWF data if necessary and become somewhat familiar.
• This being said, i'd like to merge it mainly for the purpose of pruning/cleaning branches in the repo and my mind :-).
• This can be merged directly to the downstream branch (iri-vhi-trigger-simulation) or to main after iri-vhi-trigger-simulation is merged to main
• For a little additional background info: The monthly ECMWF forecasts don't really fit into the current framework which is based on 3-month tercile probability forecasts. We are toying with the idea of calculating transforming these data to that format, but it's out of scope of this branch which is mainly just downloading.

[caldwellst]

Overall looks good! Just a few code comments but I think the download functionality works well! Good find on the R package too. All good to merge whenever you think ready, just address the comments as you see fit or not at all!