Data quality

Add a comment about data quality.

That obvious erroneous periods have been removed, e.g., instrument malfunction, and periods where instruments have been removed. Irradiance measurements have not been corrected for thermal offsets.

No file or documentation entry point is named; first inspect the weather data website content to find the section describing data quality. Add the supplied note about removed erroneous periods, removed instruments, and uncorrected thermal offsets, then verify that it appears correctly in the published content.

Written by the indexing model from the issue text.