Redesign ZeroCrossing handling ( ticket:3851)

[wibraun]

• introduce new Expression for zero crossings
• this will make the search of ZeroCrossings much easier, because currently we write the index of a
  zeroCrossing in the the involved Relation, but need to collect them both.

[hkiel]

Any progress on this topic?

[wibraun]

Unfortunately no :( there are higher prioritized tasks on my todo, and this has a nice to have status.