Support linking to multiple camera devices

[rly]

Pending a fix for hdmf-dev/hdmf#693

[rly]

The original and labeled video files (currently represented as a list but my be changed to a set from #12) is ordered. We intend the set of cameras to align with these lists, so the nth video was generated by the nth camera. However, a set of N links (quantity: '*') is unordered. There is no simple and straightforward way to align these in the PoseEstimation data type.

The proposal in #12 would change the video files to be ImageSeries objects, which already have an optional link to a Device that would represent the camera. That should suffice. So the devices field should be removed from the PoseEstimation type in the resolution of #12 and this PR will no longer be necessary.

[rly]

Already implemented in a different PR.