implement multispectral reference sensor settings

[Andreas Janz]

Currently we use Sentinel-2 as multispectral refrence sensor for calculating spectral indices and in the Raster Layer Styling panel.

The center wavelength and fwhm values are hard coded:

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It is proposed to implement a widget for selecting the sensor model. Store predefined sensors for Sentinel-2, Landsat 8, Landsat 457 and MODIS in json files.

Place the settings widget under MainMenu > Settings > Multispectral Reference Sensor.

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[Andreas Janz]

For Sentinel-2 use official data from here: https://sentinels.copernicus.eu/web/sentinel/missions/sentinel-2/instrument-payload/resolution-and-swath