improve glossary term descriptions related to "continuous-valued layers"

[Andreas Janz]

• explain how the renderer defines the default target variables

  • Graduated renderer for single-target layer
  • Diagrams renderer for multi-target layer

• update screenshots

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[Andreas Janz]

Wait for final decision on example dataset for regression.

I proposed to use Total Vegetation Cover, Building Heights and Total Material Stocks as variables.

[Andreas Janz]

We decided to use 3 different types of vegetation fractions as variables:

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E.g. use a Graduated renderer for single-target regression:

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Or a Diagrams renderer for multi-target regression:

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