Returns the fully-resolved data config (base defaults merged
with sparse overrides). If var_name is provided, returns only that
variable's config entry.
Arguments
- var_name
[character, optional] A specific variable name to retrieve. If
NULL(default), returns the entire config.- options_file_name
[character, optional] The name of the options file. If
NULL(default), the user will be prompted interactively.- options_dir
[character, optional] The directory containing options files. If
NULL(default), the default directory is used.