Added coast linestrings and outer loop for tile-grid calculation
This finishes the assembly (see function static_data_in_quadkey) of vector data by adding coastline linestrings to the tile_polygons_gdf data object. Example below:
All related code from the main function has been moved into a functiontile_data.
The next steps concern the actual construction of the tile-grids. For that I've outlined a rough structure (a loop), to where I then later add dedicated functions. For now there are only placeholders instead.