rethink having 3d vegetation in three separate files

[stefanfluck]

currently, 3d vegetation is split in forest, tree lines and single trees/bushes.
however, they are all polygons, for which LAI and shape parameters could be contained as an attribute in the file itself. This simplifies the required files significantly.

[stefanfluck]

#23 is actually the same as this one, or at least can be solved in one go