Use better geometry package for distance calculations etc.

E.g. in `ewatercycle.util.py` we have `find_closest_point`, which computes a rough estimate of distance between point and grid. However, this distance is not accurate (e.g. could also use haversine or vincenty distance), and it doesn't handle edge cases such as grids that wrap around the prime meridian, or conventions like (-180, 180) vs (0, 360). Instead of reinventing the wheel, maybe we can use something like `geopy` or another package that is tailored to this kind of problems.