function to read regional summary and output long format
read.region.summary(
dir_dt_cs,
year_wanted = NULL,
sex = NULL,
add_regional_grouping = FALSE
)
directory to e.g. paste0("Rates & Deaths_UNICEFRegion.csv")
default to null, supply e.g. 1990:2019
default to NULL, will determine from file dir
if add `Regional_Grouping` column