[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

[sbreitbart-NOAA]

This feature is going to be so useful and important! Also great to see repeated code removed in favor of making a streamlined function useful for these and future plots.

I have some questions and suggestions. This functions as expected in most ways, but I think there are a few tasks left to do- like making sure this works and is documented clearly for various situations.

[sbreitbart-NOAA]

I like the messages shown when a ref line isn't found. Here, it shows up for one model but not both, even though zero ref lines are shown (i.e., no message warning about a lack of ref line for the stockplotr example data, but it's not shown):

plot_biomass(dat = list("petrale" = dat, "skate" = skate), module = "TIME_SERIES")
! biomass_msy not found for skate
Warning messages:
1: Removed 1 row containing missing values or values
outside the scale range (geom_hline()).
2: Removed 1 row containing missing values or values
outside the scale range (geom_text()).

[Schiano-NOAA]

That's interesting because it should show for both if both don't appear. It's in a loop