Refs #10281. Removed counting variable from Group.

It was only used for testing purposes (counting number of required operations).
Refs #10281. Added simple factory function for Group-objects.

Since groups can be constructed from strings only and are probably not used directly in code so much, this solution seems simpler than a complete factory.
Refs #10281. Creating vectors of symmetry operations.
Refs #10280. Added class to deal with lattice centering