handle multiple objects in storage root with same id

[ghukill]

Take the following example of 4 objects, two sharing ids:

In [34]: for obj in sr.get_objects(): 
    ...:     print(obj.id) 
    ...:                                                                                                                                                                                                                                                                                   
93545273046d4857b30037ad4d59463f
93545273046d4857b30037ad4d59463f
4bc8bfd37c654748a9aa899a3c413e84
4bc8bfd37c654748a9aa899a3c413e84

Currently, OCFLStorageRoot.get_object():

1. takes the passed identifier
2. using storage engine, checks for object at that calculated path
3. if present, returns

In this construed example, though two OCFL objects share the same identifier, one of them is not valid in the context of the Storage Root's storage engine (storage_pair_tree in this instance). This makes this issue acutely related to #7.