[METIS] Include rectification in example notebook

[teutoburg]

The error occurs because unlike in TRACE_LSS_L_sci.fits, the new TRACE_LSS_L.fits does not define WCS keywords in the (primary) header. But maybe that's fine, the larger issue behind this seems to be that the example notebook LSS-YSO_model_simulation mentions that:

Before showing the background subtracted spectra, we convert the pixel numbers to wavelength and spatial position along the slit. This is possible because the current version of Scopesim/METIS by default uses perfectly linear mapping of the spectra onto the detector. The WCS keywords to use are in metis['spectral_traces'].meta. The plots will be restricted to the area covered by the spectra.

Which is not true anymore. So perhaps we need to include the rectification in that notebook. For now, I'm disabling the part of the notebook that causes problems, so our CI can pass again...