Collection of Intervals for OrdUnidimDenseSet #128
Description
Currently, OrdUnidimDenseSet has a collection of SePieces as member, which should be a collection of Intervals. This was necessary because Set exposes its implementation, by allowing a traversal of its pieces. When the refinement is completed, this will no longer be the case, so MDIOrdCollection _pieces can be replaced with IntervalCollection _pieces.