Move cartesian coordinate handling to a utils package

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)

Start by locating the existing cartesian conversion helpers and tie_points_interpolation entry point referenced in the issue. Review how coordinate conversion is currently handled across the package, then determine which shared utility and call sites need to use the proposed decorator. Done means cartesian handling is centralized and the affected interpolation behavior remains correct.