allow selection of a raster layer used as input in "Fit Transformer" algorithm

[Andreas Rabe]

Suggested by @Agustin Lobo and supported by the team.

The most common usecase, when using a transformer, is to use a raster layer as input and not a training dataset:

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It is proposed to move the Training dataset parameter into the Advanced options, and show a raster layer used as input instead.

[Andreas Rabe]

Also add a “Sample Size” option. Set default to 1000 samples.

[Andreas Rabe]

Done

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[Andreas Janz]

resolves #1099

→ <<cset 83af4eb7e0ec>>