turn a dataframe into a known population vector
df.to.kpvec.Rddf.to.kpvec takes a dataframe which has a column with
known population names, and a column with known population
totals, and turns it into a known population vector. if the
names of the survey variables corresponding to each known population
are available, they can be passed in as well
Value
a vector whose entries have the known population values and whose
names have the corresponding kp.var value