Speed up calculation of drainage map

[vgro]

The drainage map defines the direction of water flow across the grid based on a digital elevation model. The current implementation in 'calculate_drainage_map' is very slow and will likely not run properly with a realistic grid size, see

virtual_ecosystem/virtual_ecosystem/models/hydrology/above_ground.py

Line 202 in 450aaec

def calculate_drainage_map(grid: Grid, elevation: np.ndarray) -> dict[int, list[int]]:

The two functions find_lowest_neighbour() and find_upstream_cells(lowest_neighbours) are super inefficient. I do not have a specific solution in mind, it might need a completely different approach. It is also worth considering if this should move somewhere more central as something similar might be required for the animals.