Add broad line support

[multiwavelength]

At the moment, gleam works really well for narrow emission lines (such as those associated with narrow-line regions in AGN and star-forming regions). This is because the starting solution for the Gaussian fits is set to the resolution of the instruments. This starting solution also works for broad lines (>500 km/s), as long as the line is well separated from other lines and the fit is simple. However, the fits do not converge to the correct solution when one has both broad and narrow components of the same emission line. To fix this, we should give users the option to label lines in the spectral lines list as broad and provide a different starting solution for the Gaussian fits, in line with expectations for broad lines (i.e. a FWHM starting solution equivalent to 500-1000 km/s)

[multiwavelength]

I quickly tested this and it works BEAUTIFULLY.