Add cross-shore varying vegetation extent based on dunetoe location

[christavanijzendoorn]

I will be tackling an aspect of #49. In long-term simulations of dune development the extent of vegetation is expected to change as dunes move seaward or landward. However, the current implementation of vegetation in AeoLiS only allows for a vegetation cover that is static through time. I will create a dynamic vegetation cover by assigning the pre-defined vegetation cover to all cells that are landward of the dune toe.