Included version.py for __version__ attribute.
Included new nifty_config and d2o_config files.

Resorted the packages and the import structure.

Started rework of the d2o slicing: introduced 0-dimensional advanced indexing capabilities.

-> Many minor and major bugfixes
-> Reworked indexing (now works with negative step sizes, too)

Updated the naming of domain, codomain, target and cotarget in the spaces, fields and operators.
-> The propagator_operator is now 3 times faster