Wrap timedelta as list
requested to merge mdefende/gpfs-policy:bugfix-wrap-single-int-timedelta-as-list-when-creating-breakpoints into patch-v0.5.1
create_timedelta_breakpoints accepts either a single integer or list of integers for the delta_vals but does not wrap the single integer case as a list. This led to an exception when trying to loop through the breakpoints since you can't loop through a naked integer.
This adds the same wrapper as in create_timedelta_labels to store a naked integer in a list with a single element.