Add tests for radiation models

[Mateasek]

Radiation models should get tests making sure they provide the expected spectra and are not changed accidentally:

• [x] GaussianLine
• [x] MultipletLineShape
• [x] StarkBroadenedLine
• [x] BeamCXLine
• [ ] BeamEmissionLine

[mattngc]

This is a good idea. Will help with documentation and validation as well.

[vsnever]

I am currently adding Zeeman spectral line shapes to Cherab along with unit tests. The TestLineShapes class will also include the tests for GaussianLine, MultipletLineShape and StarkBroadenedLine, because all these models can be tested with the same plasma setup.