Examples of using kolmogorov_forcing in semi_implicit_navier_stokes

Dear researchers:

Do you happen to have any example of using [kolmogorov_forcing](https://github.com/google/jax-cfd/blob/main/jax_cfd/base/forcings.py#L63) in [semi_implicit_navier_stokes](https://github.com/google/jax-cfd/blob/main/jax_cfd/base/equations.py#L128) ?

Naively passing `kolmogorov_forcing` into `semi_implicit_navier_stokes` will not work.
Mimic the usage in [spectral](https://github.com/google/jax-cfd/blob/main/jax_cfd/spectral/equations.py#L213) will give errors like "TypeError: unsupported operand type(s) for /: 'function' and 'float'" or "ValueError: arguments have different tree structures"

Examples in https://github.com/google/jax-cfd/blob/main/notebooks/spectral_forced_turbulence.ipynb will work, but one has to manually convert vorticity into velocity.

Thank you very much!