Willow vines generation is still broken, can't fix here since moretrees uses "lazy generation"
I converted the fill ratios via (100-old_ratio)/100 (formula from biome_lib), we might want to adjust the defaults since vines are pretty rare currently :\
Next step for https://github.com/mt-mods/plantlife_modpack/issues/27
WIP
Note: Willow vines generation is completely broken (pretty sure it was already before), integration w/ moretrees needed