Walker methods
--------------

In the COPY, DELETE and MOVE methods we need to walk over
a tree of resources and collections in order to copy, delete
or move them.

The difference between all these walks is only that we perform
a different action on the resources we visit. Thus it might be
the simplest solution to provide a walker class or method to
do that work and give it a function to perform before starting.


Way of walking
--------------

When we delete things we should do it bottom up but when we copy
or move things we should create resources top down. Thus we actually
need 2 methods.

But the following method might work: We create a list of all the nodes
in the tree in tree order (means top down, left to right). When
we walk over this list from begin to end we can copy and when we
move backwards we can delete.

Thus we need an indicator for the direction and the method to
perform on it.


Here the iterative approach (in order to save memory):
dc = dataclass
queue = list = [start_uri]
while len(queue):
    element = queue[-1]
    childs = dc.get_childs(element)
    if childs:
        list = list + childs
    # update queue
    del queue[-1]
    if childs:
        queue = queue + childs


(first try..)