Refactor transfer functions to handle division by zero and NaN values

- Added np.errstate context managers to suppress warnings and handle divisions by zero in various transfer functions.
- Updated the calculation of the `zo` variable to return np.inf when mu is zero to avoid division by zero errors.
- Modified the `_xis` function to return NaN for non-finite values.
- Enhanced unit tests to ensure proper handling of edge cases and validate output shapes for both no-slip and free-slip boundary conditions.
- Renamed test methods for clarity and consistency with the new transfer function implementations.

Author: selipot