multiwavelength/gleam: Initial release of gleam

Initial release of the gleam software. Main features include:

• Process large numbers of sources in batch mode.
• Jointly fit lines located close together, report upper limits and identify lines without spectral coverage.
• A single configuration file for an entire project, which you can use to customize the fitting for specific telescope/instrument configurations or even single sources.
• Human-readable YAML configuration file, in which units can be specified.
• Plots of an entire spectrum with the line fits, as well as plots for each individual line fitted.
• Well integrated with Astropy, which enables the use of units and fits tables.
• Uses LMFIT to perform the fitting and report errors on fit parameters.
• Simple installation with pip.