Add navigation links between time spans and transitions in TimeSpanMap
The TimeSpanMap container provides duals views to its data.
The Span view represent the time span during which one entry is valid, it has a value, and start and end dates.
The Transition view represent one transition date between two spans.
The Span view can be retrieved only for single dates, using map.getSpan(date), it is not possible from a span to get naturally the previous or next span, one has to use dirty tricks like map.getSpan(currentSpan.getStart().shiftedBy(-0.001)), hoping the previous span is larger than 1ms.
The Transition view can be retrieved only globally as a NavigableSet<Transitions> using map.getTransitions(). There is
no way to get the Span on each side, only the value contained in the Span.
The dual Span and Transition views are therefore mutually exclusive at API level, despite they are intertwined in the implementation.
It would be nice to have a way to get the Transition at start and end of a Span, and to have the before and after Spans at both sides of a Transition. It would also be nice to have next and previous methods to navigated from one Transition to the other or to one Span to the other.