Move cartesian coordinate handling to a utils package

[sjoro]

Cartesian coordinate handling could be generalized in a utils-package, something like:

from functools import wraps

def use_carts(use_carts=False, latitude_xy_threshold=60):
    """Convert input to a function to cartesian and back."""
    def decorator(func):
        @wraps(func)
        def wrapper(data, **kwargs):
            if use_carts:
                data = _lonlat2xyz(*data)
            result = func(data, **kwargs)
            if use_carts:
                result = _xyz2lonlat(*result, latitude_xy_threshold=latitude_xy_threshold)
            return result
        return wrapper
    return decorator

Usage would be simplified to

interpolate = use_carts(to_cart)(tie_points_interpolation)
return interpolate([longitude, latitude], scan_alt_tie_points, tie_points_factor)