Add better support for segmented LUTs

[CPBridge]

#139 added support for segmented LUTs in presentation states, however the user is left to themselves to construct valid lut data, which is not straightforward.

We should provide utility methods or alternative constructor to allow users to construct LUTs in a more intuitive way

[hackermd]

I generally agree. My only concern is that there may not necessary be an easier way, but just different ways. And introducing another way to describe the segmentation may be even more confusing.