Added more preparation spatial calculations
Quite lengthy (and time intensive) MR that adds a comprehensive chain of spatial operations to determine those level 18 tiles in a working quadkey, say a level 9 one, that are at the coast. The result is a GeoDataFrame (gdf) of level 18 quadkey coast tiles and the respective water and land polyons per such tile. The resulting gdf is supposed to be the basis for the next calculation steps. Validity of new functions is ensured with unit tests.
Will update with explanations on the intermediate steps and data.