Define an interface for keypoint models and wrap the current best models

[cotemyriam]

Define common interface for keypoint models. Wrap current best models (Pierre Luc's and Vicent's from my course + current best model from Yoshua's course). Keep researching better models.

[ Xavier B. and Pierre-Luc; synchronize with Pascal V.]

Various ideas for improving models:

combine with a-priori probability model of keypoint location (not trained on images, but keypoint locaitons only)
consider other things.