t.sentinel.import: Add option to add a band offset after importing S-2 data

[griembauer]

Since Sentinel-2 Processing Baseline 4.0.0 a systematic offset has been introduced to all reflectance values. This offset is currently 1000 and can be read from the scene metadata if available.

In this PR, an option to indicate an offset number is given to the user. The offset is then added to all imported bands, and via a small hack, the description.json that holds band metadata that is relevant for e.g. i.sentinel.mask is preserved.

Important:
An ideal solution would obviously be to automatically parse the offset from the scene metadata and implement the offset addition directly in i.sentinel.import. This is not done for the following reasons:

• In the current structure of i.sentinel.import, the metadata is read after importing the data itself - changing this would be a major refactoring of the module
• The offset is only to applied for actual reflectance bands. In this PR, it is ensured that it is not applied to cloud and cloud shadow bands, however for the remaining bands imported by pattern, it is up to the user to make sure that only relevant bands (and not quality layers etc.) are imported. For i.sentinel.import, a blind application of the offset would not be a valid solution, so an internal mechanism would have to make sure that only reflectance bands are affected - again this requires a major refactoring
• as indicated in the corresponding grass addons issue, if a major refactoring of i.sentinel.import is foreseen, then it would make sense to directly make use of the GDAL Sentinel-2 driver.

--> in essence, this PR represents a small-effort workaround for an issue that would require much more effort for a sustainable solution

[neteler]

Suggested change

	Since Sentinel-2 <a href="https://forum.step.esa.int/t/info-introduction-of-additional-radiometric-offset-in-pb04-00-products/35431">Processing Baseline 4.0.0</a>, a systematic offset has been introduced to all reflectance values.
	To account for this, the <b>offset</b> option allows the user to indicate how reflectance data should be corrected (e.g. -1000).
	Since Sentinel-2 <a href="https://forum.step.esa.int/t/info-introduction-of-additional-radiometric-offset-in-pb04-00-products/35431">Processing Baseline 4.0.0</a>, a systematic offset has been introduced
	to all reflectance values.<br>
	To account for this, the <b>offset</b> option allows the user to indicate how reflectance data should
	be corrected (e.g. -1000).

(break long lines, see https://github.com/OSGeo/grass/blob/main/doc/development/style_guide.md#markup-style-guide - "More notes on markup")

[neteler]

Suggested change

	# % description: Offset to add to the Sentinel bands to due to specific processing baseline (e.g. -1000)
	# % description: Offset to add to the Sentinel-2 bands to due to specific processing baseline (e.g. -1000)

Perhaps only S2 related?