Function to calculate the quad-tree for an entire feature
This MR adds two changes. Geometries of the vector_buffer object are now "packed" into MultiGeometries in the classify_coast_buffer function, making it a single MultiLineString coastline instead of a multitude of LineString coastline segments, which in return allows to only once execute the buffer operation on the coastline, instead of a several 100 or 1000 times. Thus also eliminating overlaps between buffers, which eliminates the need to sort out duplicates in buffered tiles and other potential sources of errors.
The second addition is function quadtree_grid. While function tile_quadtree from !20 (merged) creates a quadtree from a single tile (or a tuple of complementary tiles), quadtree_grid goes along a set of tiles (e.g. the level-18 tiles that describe the coastline) and using tile_quadtree to create the quadtree structure for an entire feature within a larger tile. This quadtree (see picture below) will then in a next step be imported as tiles into the tile-database.
Pink: aggregated tile-grid, Blue: coasltine linestrings