Add order_children methods in Diff Object

[dgarros]

fixes #12

This PR restructure the get_children method in the Diff object to leverage new order_children methods.
By default get_children will check if a order_children method specific to each children is available (order_children_site for site) or call the order_children_default.

Each order_children method needs to return an Iterator of DiffElement and get_children is returning a global iterator with all children.

Here is an example on how to order the sites in alphabetical order

class MyDiff(Diff):

    @classmethod
    def order_children_site(cls, children) ->  Iterator["DiffElement"]:
       """Return the site children ordered in alphabetical order."""
       keys = sorted(children.keys(), reverse=False)
        for key in keys:
            yield children[key]

a = BackendA()
a.load()

b = BackendB()
b.load()

diff_a_b = a.diff_to(b, diff_class=MyDiff)

[glennmatthews]

With the API restructuring made in #10, would it be more consistent (and make any sense) to make the ordering into a method on the individual DSyncModel subclasses instead of a method of the DSync class? Maybe even let the DSyncModels define __lt__ and __eq__ and use sorted(self.children[group])? Just thinking aloud here.

[dgarros]

With the API restructuring made in #10, would it be more consistent (and make any sense) to make the ordering into a method on the individual DSyncModel subclasses instead of a method of the DSync class? Maybe even let the DSyncModels define __lt__ and __eq__ and use sorted(self.children[group])? Just thinking aloud here.

I thought about it too but since the ordering is currently happening in the Diff object I don't think we can have access to the DSyncModel within the get_children method, please let me know if I'm missing something.