improve "Save spectral library as JSON file"

[Andreas Janz]

• use GeoJSON to be recognised as a valid vector layer
• make sure that the GeoJSON is concidered a valid EnMAP Spectral Library

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[Andreas Janz]

Also, place “Save as GeoJSON“ context menu options here:

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And here:

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[Andreas Janz]

FYI @Benjamin Jakimow , after #1044 is implemented, we may remove this algorithm and the context menu options again.

[Andreas Janz]

resolves #1045

→ <<cset 2db9f29df2da>>