overhaul Spectral Index Creator app

[Andreas Janz]

• replace widgets for band mapping and constants by QTableWidgets

• replace spectral indices list with more structures tab widgets known from GEE TSE app:

  

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• allow user-defined Spectral Indices

  

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[Andreas Janz]

Done, GUI looks like this:

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[Andreas Janz]

The user can add his own indices by editing this file:
\enmap-box\enmapboxprocessing\algorithm\createspectralindicesalgorithm.other.json

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We may have a better solution later, that doesn’t require editing a source file.

[Andreas Janz]

resolves #968 and #993

→ <<cset 7e9f3eed8f5c>>