@astrofrog  No, it was real. There was a problem with python 3 - some of the functions in the operator module are moved to the numbers module in python 3.
Anyway, I think I addressed all your comments. Previously you suggested to move the WCS related models into a wcs subpackage. I don't always see a clear distinction - some of the general models can be used for WCS purposes. Instead, how about pulling all base classes and classes which set up the framework in general into a separate module, "core.py", and keeping the rest of the models in models.py. In the future, when we have more models, we can rearrange the modules again.

I had very good comments and the documentation improved significantly. But I really think the only way we can know if this is useful is to start using it with real data. I'm sure this will bring also other documentation shortcomings.