AirSensor2 0.6.0

• Added OpenAQ integration for creating monitor objects from OpenAQ data.
• New OpenAQ_createLocations() for discovering available locations with filtering and lookback support.
• New OpenAQ_createMonitor() to generate AirSensor monitor objects with a guaranteed hourly time axis.
• New OpenAQ_downloadRawData() with built-in pagination, rate limiting, and multi-sensor handling.
• Added parameter and provider helper functions for working with OpenAQ metadata.
• Implemented caching of OpenAQ metadata to improve performance.
• Updated pat_applyCorrection() to latest EPA correction algorithm.
• Added AirGradient_applyCorrection().