[feat]: enable user to add a reference line to the plot for each input dat

[Schiano-NOAA]

I moved the reference line portion of the plot into it's own function -- add_reference_line() -- which gets added to the plot using the (+) operator.

This feature automatically applies to each input dat. There are 4 conditions that this feature handles

1. reference lines and values extracted for each input dat. e.g.
   plot_spawning_biomass(dat=list("mod1"=mod1, "mod2"=mod2), ref_line = "msy") SBmsy is found for each model.
2. custom values are input by the user for each model in order they are listed in dat. e.g. plot_spawning_biomass(dat=list("mod1"=mod1, "mod2"=mod2), ref_line = c("msy"=1000, "msy"=1200)) In this instance, any name or combinations can be used as long as there is one value for each element in the dat list.
3. different reference lines for each model extracted automatically. e.g. plot_spawning_biomass(dat=list("mod1"=mod1, "mod2"=mod2), ref_line = c("msy","target"))
4. Don't include lines at all. Set ref_line = NULL

[github-actions]

New version checklist

• Package version in DESCRIPTION has been updated
• Release notes have been drafted/published
• Cheatsheet content has been updated (if applicable)
• Cheatsheet version has been updated