Save object id's as part of links and references

[oruebel]

• To enhance portability of links and references it would be nice to store the object_id of the Zarr file in addition to the relative path when a link/reference points to an external file. This will be useful both for error checking but can also help resolve links in case that path's are not valid

[oruebel]

Implementing this will require:

• Updating the ZarrReference class to add a source_object_id field
  

  hdmf-zarr/src/hdmf_zarr/utils.py

  Line 228 in 6841196

  class ZarrReference(dict):

• Updating ZARRIO.__get_ref to populate the object_id of the file
  

  hdmf-zarr/src/hdmf_zarr/backend.py

  Line 478 in 6841196

  def __get_ref(self, ref_object):

• Updating tests
• Updating the storage documentation accordingly https://github.com/hdmf-dev/hdmf-zarr/blob/dev/docs/source/storage.rst#links

[oruebel]

#57 partially addresses this issue by storing the object_id of the referenced object itself if it is available. However, not all links may point to object with an assigned object_id, so there are cases where the object_id will be None.

Because of this, storing the object_id of the source Zarr file will be useful in addition as well. However, determining the object_id of the source is trickier because we only have the path of the source object and the builder of the referenced object. To determine the object_id of the source it seems that we would then either need to open the source Zarr file, or at least open the JSON file with the attributes of the source Zarr file. This approach is not ideal, as we should avoid opening other files during write. Alternatively, we may be able to trace the parents of the builder until we find the the root builder (i.e., the builder where builder.parent is None should be builder of the target Zarr file).