Das Gupta equation 6.11: Standardises rates across populations
dg611(srates, all_p, y, factor)
data.frame object including K-a standardised rates for each population for given factor a, across N populations
a dataframe/tibble object of standardised rates from dg2pop
character or numeric vector of all N populations
character/numeric indicating a single population
string indicating rate-factor being standardised.