Update

• Add new iso8601_to_strings() function superseding numeric_to_extended_iso8601() and iso8601_to_dates()
• Add new iso8601_to_period_labels() function for converting ISO8601 dates to period labels (e.g., "2022q1" or "2022m9")
• Help text as markdown document: improved formatting and structure
• Raise min. Gretl version to 2023a
• Bugfix: refactor datetime_components() function to improve variable declarations and add type safety