This function contains a copy of the code in
cut.default which determines the
numerical breakpoints used to convert x to a factor.
It returns the breakpoints only.
The arguments x and breaks have the same interpretation
as in cut.default. Only the range of x
is used in the computation, so x could be replaced by range(x).
This function would normally be used when breaks is a single
integer specifying the number of intervals for the cut operation.
It returns the exact numerical values of the breakpoints which are
determined, but not returned, by cut.default).