CQuad does not order actions based on error estimates

The current implementation (IIRC) is meant to replicate the behaviour of Christoph's code in the case of sequential execution.

Going forward, we should improve this. Currently, if there are any intervals in priority_split, then they are split first. However, priority_split is itself not an ordered collection, so the intervals inside it are ordered in an arbitrary way with respect to the expected loss improvement when splitting them.