class TGeoIterator

 TGeoIterator

 A geometry iterator that sequentially follows all nodes of the geometrical
 hierarchy of a volume. The iterator has to be initiated with a top volume
 pointer:

    TGeoIterator next(myVolume);

 One can use the iterator as any other in ROOT:

    TGeoNode *node;
    while ((node=next())) {

    }

 The iterator can perform 2 types of iterations that can be selected via:

    next.SetType(Int_t type);

 Here TYPE can be:
    0 (default) - 'first daughter next' behavior
    1           - iteration at the current level only

 Supposing the tree structure looks like:

 TOP ___ A_1 ___ A1_1 ___ A11_1
    |       |        |___ A12_1
    |      |_____A2_1 ___ A21_1
    |                |___ A21_2
    |___ B_1 ...

 The order of iteration for TYPE=0 is: A_1, A1_1, A11_1, A12_1, A2_1, A21_1,
 A21_2, B_1, ...
 The order of iteration for TYPE=1 is: A_1, B_1, ...
 At any moment during iteration, TYPE can be changed. If the last iterated node
 is for instance A1_1 and the iteration type was 0, one can do:

    next.SetType(1);
 The next iterated nodes will be the rest of A daughters: A2,A3,... The iterator
 will return 0 after finishing all daughters of A.

 During iteration, the following can be retreived:
 - Top volume where iteration started:    TGeoIterator::GetTopVolume()
 - Node at level I in the current branch: TGeoIterator::GetNode(Int_t i)
 - Iteration type:                        TGeoIterator::GetType()
 - Global matrix of the current node with respect to the top volume:
                                          TGeoIterator::GetCurrentMatrix()

 The iterator can be reset by changing (or not) the top volume:

    TGeoIterator::Reset(TGeoVolume *top);

 Example:

 We want to find out a volume named "MyVol" in the hierarchy of TOP volume.

    TIter next(TOP);
    TGeoNode *node;
    TString name("MyVol");
    while ((node=next()))
       if (name == node->GetVolume()->GetName()) return node->GetVolume();