Validate Kolmgorov stepper

[Ceyron]

Ensure the spectrum is comparable in the validation/validate_kolmogorov_spectrum.ipynb notebook. It could be that these lines

exponax/exponax/nonlin_fun/_vorticity_convection.py

Lines 168 to 176 in 32a79b2

	self.injection = jnp.where(
	injection_mask,
	# Need to additional scale the `injection_scale` with the
	# `injection_mode`, because we apply the forcing on the vorticity.
	-injection_mode
	* injection_scale
	* build_scaling_array(num_spatial_dims, num_points, mode="coef_extraction"),
	0.0,
	)

cause a wrong scaling in the injected energy. This could be related to #34

[Ceyron]

@GeorgKohl also worked with this stepper. Linking him here for reference.