Update implementations of INode methods

Particularly, some derivative classes have obsolete or missing nodeChildren() function.

added Core cleanup refactoring labels

removed refactoring label

If child class X has no X::nodeChildren, then INode::nodeChildren is used.

Which functions are obsolete?

For example, ISimulation::nodeChildren() doesn't mention m_background.

ScatteringSimulation should also return m_beam and m_detector but nodeChildren() is not overriden there.

SpecularSimulation contains m_scan but has no its own nodeChildren().

The general impression is that division of class members on INodes and other types is arbitrary and thus on updating the class it is easy to forget to create or update nodeChildren method.