Avoid * imports, explicitly declare exported symbols

[Anthchirp]

less from x import *
more __all__=(...)

[Anthchirp]

This pull request is not meant to be merged straightaway, but as a basis to discuss possible import * and __all__ conventions.

• The best way for automatic linting and thus the way that makes flake8 the happiest is to always avoid import * and to declare __all__. This means that if you want to export symbols that come from C++ you will have to declare them twice. This is annoying if you need to add or remove symbols.
• On the other hand it does make your API very clear: This function I imported from C++ is supposed to be made available through here.
• One can argue that files that only do this, and do not contain any actual Python code can be cleaner by having a import * and declare an __all__. This would be mostly acceptable to flake8 and avoid duplication in the code. One example is format/image.py in this pull request.
• Whereas if your file also contains wrangling code (eg. imageset.py) it makes sense to not do import * to make it clear where symbols come from, and declare __all__ to make clear where symbols go to. For example the different *Aux classes do not need to be exported.

[Anthchirp]

unless someone shouts will (squash) merge this shortly

[Anthchirp]

no complaints => merge