DRYify `get_map_data()` and `get_forecast_data()`.

[dylanhmorris]

We should DRYify get_map_data() and get_forecast_data(). There is a lot of repeated code between the two. In fact, I believe the get_map_data() table may just be the get_forecast_data() table filtered to the Hub ensemble.

Proposal

Split table creation into

1. a function to generate the table (as a tibble) for all or a subset of models. Call this function summarize_ref_date_forecasts().
2. a light wrapper that generates a table using summarize_ref_date_forecasts() and then saves it to disk at a given path. Call this write_ref_date_summary().
3. Two convenience functions named write_ref_date_summary_ensemble() and write_ref_date_summary_all() that replicate the current functionality of get_map_data() and get_forecast_data(), respectively

@O957 does this make sense to you?

[O957]

Do you want me to do this in a separate PR or in #39 ? Right now I am pursuing it in the same PR.

[dylanhmorris]

My suggestion in #39 was to merge as-is and then pursue #70: #39 (review)