Adding way to uniquely hash OpenSim models to `engine.py` to support downloading only data with a given model

[keenon]

Soon, we'd like the CLI to support addb download --models_like <my_model.osim path>, but to support this we need some way to identify when OpenSim models are "the same". This is hard, because we can't just has the OSIM file text, cause it may have different whitespace or markersets or whatever and still have the same DOFs, and be functionally equivalent. But, it's possible to have the same number of DOFs with the same names, and have a different definition for the spline for "walker_knee_r"... So I dunno. But an interesting challenge.