Helper function: given a data frame, and strings for label_var, lat_var, and long_var, the df is checked for
.validate_custom_locations(df, label_var, lat_var, long_var)A data.table with standardized columns
location, latitude, and longitude.
A data.frame containing label and coordinate columns.
Character scalar naming the label column.
Character scalar naming the latitude column.
Character scalar naming the longitude column.