reference rdhs::model_datasets internally

[jeffeaton]

Description

If rdhs is not loaded in the global namespace, then create_new_filenames() throws an error:

Error in strsplit(data$FileName, ".", fixed = TRUE) : 
  object 'model_datasets' not found
> traceback()
10: strsplit(data$FileName, ".", fixed = TRUE)
9: lapply(., function(x) x[1])
8: unlist(.)
7: strsplit(data$FileName, ".", fixed = TRUE) %>% lapply(function(x) x[1]) %>% 
       unlist()
6: create_new_filenames(model_datasets)
5: private$check_available_datasets(dataset_filenames, download_option, 
       reformat)
4: client$get_datasets(dataset_filenames, download_option = download_option, 
       reformat = reformat, all_lower = all_lower, output_dir_root = output_dir_root, 
       clear_cache = clear_cache, ...)
3: rdhs::get_datasets(hrd)

This patch references model_datasets with rdhs::model_datasets to fix this.

[OJWatson]

LGTM @jeffeaton - Sorry for delay in picking this up.