Non-crystallographic symmetry operations

[sauliusg]

The space group symmetry operation descriptions (#464) apply to the whole crystal and not to a single molecule or atom cluster. For this reason they can only contain crystallographic symmetry elements and are not applicable to non-periodic structure models.

A separate property, specifying arbitrary orthogonal transformation matrices (4x4 Seitz matrices) should be standardized to describe both non-crystallographic symmetry and non-periodic molecule symmetry.