Behaviour on double-underscore Python attributes

[alexbarcelo]

AFAICT, "private variables" will break given that there is a little big of mangling when registering classes.

As of the time of writing this issue, original hierarchy of classes is lost on class registration, and thus the name mangling performed by Python should be inaccurate --and potentially result in name clashing, e.g. when using mixins.

This may be automatically resolved once bsc-dom/dataclay-packaging/issues/7 is addressed. But maybe it's worth taking that into account during the design phase of that.