Add __all__ for core part

[justusschock]

Fixes #725

Description:
Adds __all__ to each source file to determine what should be imported, when users do from package.x import *

Check list:

• New tests are added (if a new feature is added) [Not Necessary]
• New doc strings: description and/or example code are in RST format [Not Necessary]
• Documentation is updated (if required) [Not Necessary]

[vfdev-5]

@justusschock LGTM, thanks !