Some situations caused CD to produce Math Processing Errors (see #1691 (closed) for reduced test case). This was caused by the getNode() method, which did not descend into all the children. This PR fixes the problem by using a breadth-first search in getNode(), but stopping at nodes with ids, as these are other rendered MathML nodes (we only want to find nodes that are "direct" children of the current element, not similarly named ones inside nested elements).
Resolves #1691 (closed).