Finish H3.6M support

[qxcv]

Data loader is up, but I need to make the rest of the pipeline run as well. Things to do:

• [x] Make sure that loader actually runs
• [x] Define a configuration for H3.6M. The hard part is defining my subpose model, as that may require introducing some pseudo-joints.
• [x] Update pretty much anything that uses <datum>.image_path to use readim instead. cached_imflow and imflow are the main offenders there.
• [x] Fix whatever else needs fixing. Not sure what that will be right now.