chore: convert joins in mapping functions to a single modularized function

[voglerdaniel]

Jihyeon has written a standalone function to execute merges between datasets and shapefiles. As of PR #56, map_chloropleth() and map_points() execute joins like this on their own, without referencing Jihyeon's function. Eventually we should make this merge function consistent across our codebase.

[atkissoncj]

Not done