combine arg no longer used
more readable with negative sign added later
handle 0 row results
handle 0-row data frames
handle NULL edge cases
don't go too low with N ever
switch to using Hmisc instead of hydroPSO for weighted quantiles
use counts from plyr, skips 0 counts
make this work independent of number of grouping cols
renamed
swap hydroPSO dep out for Hmisc
catch the null case
add column selection as vectors
even safer cbind for 0 col data frames
renaming quantile.cols