Add data structures for multi-view/3D

[talmo]

We should implement the core new data structures being added as part of SLEAP3D.

Checklist:

• RecordingSession
• Camera (Camcorder?)
• CameraGroup
• InstanceGroup

Refs:

• Gist for exporting to HDF5
• MV Stack PRs

[talmo]

Implemented in #151 #153 #158