Added functionalities for the analysis of emission lines

[tomasstolker]

• The EmissionLine class has been added to analysis. The class can be used for the characterization of emission lines. Currently, there are methods available for modeling the continuum and fitting a Gaussian function to the (continuum-subtracted) line profile.
• The posterior distributions are sampled with UltraNest and can be visualized with the plot_posterior function.
• The documentation includes a notebook tutorials that explains the usage of EmissionLine.
• The read_util.gaussian_spectrum utility function has been added and returns a ModelBox.
• All fitting routines that make use of nested sampling store the ln-evidence as attribute of the samples in the database.
• Added the (optional) ln_evidence attribute to the SamplesBox.
• Some additional maintenance to the documentation pages.