Segment: make it possible to provide boundaries, not only intervals

[Mintas]

In some use-cases we have boundaries as list of points like [0, 10, 21, 55, 365] (peak positions, breakpoints, etc.), to make mir_eval.segment applicable we have to turn those points into intervals. But one can notice, that some of the evaluation methods actually convert intervals back to boundaries.

I suppose, that would be convenient to have api allowing users to avoid unnecessary conversions.