Test backwards compatibility with 0.1.0/0.1.1 PoseEstimation object

[rly]

The PoseEstimation data type (a group) in ndx-pose 0.1.0/0.1.1 has optional datasets nodes and edges. These were moved into a Skeleton data type (a group) in the ndx-pose main branch (to be 0.2.0), and PoseEstimation now has an optional link to a Skeleton object.

• Ensure that old data can still be read with ndx-pose
• Add tests here: https://github.com/rly/ndx-pose/blob/main/src/pynwb/tests/back_compat/test_read.py
• Create PR in https://github.com/DeepLabCut/DLC2NWB to work with the new format