Feature/tilegrid class
This adds a TileGrid class to provide tile-grid related functions. The line of inheritance goes BasicClass -> SpatialFunctions -> Quadtile -> TileGrid. So, TileGrid is supposed to be the place where all operations on tile-grids and with such grids happen.
As a first major function TileGrid implements classify_coast_buffer. This functions determines near-coastline level 18 tiles and sets their classification according to the bitmask using a buffer around the coastline.
E.g. coming back to this land geometry from previous MRs:
Among others, classify_coast_buffer determines and classifies the following data:
A tile-grid of water tiles in a buffer around the coastline (The four tile hole to the left is where the spit of land from above is situated.):
Coastline tiles, i.e. tiles that intersect with land and with coast (that's basically the same as above with the cut out land parts):
