If sum.abund == TRUE, then a vector of the same length as dens.idx
representing the predicted abundance for the density values in each column.
If sum.abund == FALSE and the length of dens.idx is 1,
then a numeric vector with the predicted abundance of each prediction polygon of x.
If sum.abund == FALSE and the length of dens.idx is greater than 1,
then a data frame with length(dens.idx) columns of the predicted abundance of prediction polygons