consistenly name the old "field" now "layer" in all variables and methods to avoid gehirnknoten, leverage resolved gehirnknoten to add a mapping layer lid (= layer id) to internal data which might be shared between layers (values, geoms, rasters, grids). This now avoids data duplication across layers which share the same geom field or value field or raster field and allows us to add arbitrarily many differently styled layers for the same column without any runtime impact (other than the mapping indirection, but this is just a small vector lookup)

Author: patrickbr