IASI L02 format and generic methods for flexible size formats

[lupemba]

Done

• Rename RecordChunks to RecordLayout
• Add basic functions to handle flexible size records
• Add IASI L2 format + documentation
• Add default missing values and add masking value option
• Update documentation page and add MetopDataset section

Left for another PR

• Add the "PRESSURE_LEVELS_TEMP" and other useful GIARD variables.