Resorted the packages and the import structure.

-> Many bugfixes and refactorings.

-> 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

Reworked init of d2o.
Implemented a distributor cache.

Implemented interface for slicing methods in slicing distributors. fftw_distributor is now a special case.
Implemented flatten routines for d2o.