Update Spectroscopy #3 tutorial

[kelle]

https://learn.astropy.org/tutorials/3-Trace_Extract_Wavelength-CalibrateSpectrum.html

Needs specreduce and specutils functions:
https://specreduce.readthedocs.io/en/latest/extraction_quickstart.html#tracing
https://specreduce.readthedocs.io/en/latest/wavelength_calibration.html
https://specutils.readthedocs.io/en/stable/fitting.html