Core: polyhedral form factors: refactored to prepare for deeper changes

Correctly handle kz = 0 and kz -> 0 for polarized computations

Core correct typo "asymetry"

CPack: remove STGZ

STGZ seems to be no longer supported by CMake.
We don't know what it is and why it was there in a first place.

This resolves http://apps.jcns.fz-juelich.de/redmine/issues/2216.