add yo forecast, update docs, vignettes

[bozaah]

This pull request adds support for the Norwegian Meteorological Institute (METNO) Weather API. It includes new functions for fetching and aggregating METNO forecast data, updates to documentation and examples, and updates to API key handling and validation. The changes enable users to access and process 9-day weather forecasts from METNO, improve API key management, and bring the package documentation and infrastructure up to date with the new features.

Key changes:

METNO API integration:

• Added new functions get_metno_forecast, get_metno_daily_forecast, metno_get_dominant_symbol, metno_resample_data_table, and metno_timeseries_to_data_table for accessing and processing METNO weather forecast data.
• Implemented daily aggregation of METNO forecast data in get_metno_daily_forecast, providing users with summarised daily weather information.

API key handling and validation:

• Updated get_key() to support the "METNO" service and added .set_metno_key() for user guidance.
• Added .is_valid_email_metno_api_key() to validate that the METNO API key is a valid email address, as validation for the SILO API key.

Other updates:

• Updated README and documentation to describe METNO as a data source, updated API key setup instructions, and provided new usage examples for METNO functions.
• Added new global variables for METNO data processing to avoid R CMD check notes.
• Updated NAMESPACE to export new METNO-related functions and import required packages (jsonlite, lubridate, crul).
• Added METNO-related files to .Rbuildignore for proper package building.
• Added a null-coalescing operator %||% for cleaner default argument handling in internal functions.

[adamhsparks]

Nice! I'm impressed by the vignette. You've put a lot of work into this, it's a nice addition. Well done!!!