Point group determination from trigonal space groups in hexagonal setting does not work

[MichaelWedel]

The construction of PointGroup-objects is based on space group symbols. This causes problems for some trigonal cases, because so far trigonal space groups are registered with hexagonal axes.

For fixing these ambiguities I want to introduce an additional property for Group, which contains the axes system. It can be either hexagonal or orthogonal (which covers all cases except hexagonal). This can then be used in the construction of the correct point group.

It should be easy to determine the system by inspecting the matrices of the contained symmetry operations, they have some special properties for hexagonal.

[MichaelWedel]

This issue was originally trac ticket 11432