Backport 2.1 API changes to stable 2.X series

Created by: gcarleo

Version 2.1 is introducing batching (among other things), and this results in a (small) but necessary change in the APIs of machines and samplers. Since we are still in beta for version 2.0, we should backport these API changes to 2.0 and do a release candidate for 2.0.

In practice, API changes in 2.0 should happen only at the level of the python bindings, and hopefully never touch the C++ code. I can try to do that soon.