map_unit: Map point estimates for units in a plot list.
In sakrejda/pager: Spread ggplots over multiple pages

For each plot in .pl, add a plot IFF the grouping factor in 'data' has a matching level (when used with 'as.character').

1	map_unit(.pl, data, map, unit, value, group, ...)

`.pl`	plot list, generate with 'plot_list' and pipe in.
`data`	data for point estimate plots
`map`	list of map df objects to merge data to
`unit`	bare name of the unit column (in map and data)
`value`	bare name of the value column.
`group`	bare name of the variable to use to split plots on.