Data Model: Add informational attribute processingLevel to ProductClass

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For a product class it shall be possible (but optional) to define a processing level. The following processing levels shall be defined:

• L0 (level zero): Reconstructed, unprocessed instrument and payload data at full resolution, with any and all communications artifacts;
• L1 (level one): Data derived from level 0 at the instrument’s full time/space resolution, including supplemental information needed for further processing; the generic level L1 shall only be used, if none of the more specific levels L1A, L1B, L1C is applicable;
• L1A (level 1A): Reconstructed, unprocessed instrument data at full resolution, time-referenced, and annotated with ancillary information, including radiometric and geometric calibration coefficients and georeferencing parameters;
• L1B (level 1B): Level 1A data that have been processed to sensor units;
• L1C (level 1C): Further processed data at sensor units (e. g. orthorectified data);
• L2 (level 2): Data derived from Level 1 data that have been processed to geophysical quantities of interest, e.g. wave height, cloud fraction, aerosol optical depth or radiation at the top of atmosphere, possibly including uncertainty or quality estimates; the generic level L2 shall only be used, if none of the more specific levels L2A, L2B, L2C is applicable;
• L2A (level 2A): Level 2 data in a first or intermediate processing stage;
• L2B (level 2B): Further processed level 2 data;
• L2C (level 2C): Sometimes referred to as "level 2 cumulative", i. e. aggregation of level 2 data over a larger time frame;
• L3 (level 3): Variables mapped on uniform space-time grid scales (resampling), usually with some completeness and consistency; some sources (e. g. [3]) differentiate further into level 3A, 3B etc.;
• L4 (level 4): Further derived products, e.g. climate indices, obtained from analysis of multiple lower-level products.

Justification: Informational attribute requested in ESA's Sentinel-5P Product Unit Definition and Metadata ICD (ESA-EOPG-EOPGC-SP-4, issue 1.4; a similar attribute will probably be requested for other Sentinel missions). In the framework of the Sentinel missions at least the following processing levels are used: L0, L1, L1A, L1B, L1C, L2, L2A, L2C. Processing of L3 products is planned for Sentinel-5P.

Sources: [1] https://science.nasa.gov/earth-science/earth-science-data/data-processing-levels-for-eosdis-data-products/ [2] https://cera-www.dkrz.de/docs/PostprocessingLevelDescriptions_for_CERA.pdf (based on [1]) [3] http://uregina.ca/piwowarj/Think/ProcessingLevels.html [4] https://sentinel.esa.int/web/sentinel/sentinel-technical-guides

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Data model changed and services including PRIP updated.

Display of attribute in PRIP to be tested.

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Attribute is presented in PRIP API as expected: