from_DSI_to_PBox

[rospoly]

I need to do some work on the method from_DSI_to_PBox.
In particular, in case of redundancy (e.g. when a ds element has 0 prob mass) we cannot remove the element, instead, we have to MERGE it with a right/left ds element.
The reason is, even with 0 prob mass, the interval might be wider than the left/right ds element, thus if we remove it we affect our soundness.
Same thing when the cdf_up is 1, we do not remove the remaining ds elements, we merge them to keep their intervals.