Create a factor dissecting the range of a numeric variable into
bins of equal width, (roughly) equal frequency, or at "natural"
cut points. The cut function is used to create the factor.
bin.var is a synomym for binVariable, retained for backwards compatibility.
one of "intervals" for equal-width bins;
"proportions" for equal-count bins;
"natural" for cut points between bins to be determined
by a k-means clustering.
labels
if FALSE, numeric labels will be used for the factor levels;
if NULL, the cut points are used to define labels; otherwise a character
vector of level names.