comtradr 1.0.5

New Features

• Added available_variables to ct_get_ref_table() to query comprehensive
  metadata about all Comtrade API variables. Returns variable descriptions,
  original API field names, and tidy column names in a single table. This makes
  it easy to:
  • Understand what each column in trade data represents
  • Create data dictionaries for documentation
  • Map between original and tidy column names
    Example: ct_get_ref_table("available_variables")

Minor improvements

• Enhanced documentation for reference tables
• Added comprehensive tests for variable metadata functionality and ct_perform_request()