Problem 2457

The signature for `G4MultiUnion::AddNode` is 
```
void AddNode(G4VSolid& solid, G4Transform3D& trans);
```

This prevents compact construction via in-place declaration of the transform like
```
munion->AddNode(*solid, G4TranslateX3D(dx));
```
and instead requires declaring an extraneous transform object. In the implementation, the transform is copied into a member vector, so there is no reason not to make method argument `const`. 

Related, it would be nice to provide signature that accepts the solid as a pointer
```
void AddNode(G4VSolid* solid, const G4Transform3D& trans);
```
to be more similar to the boolean solid constructors.