Refactoring/get miller indices

Finally got the skin of UnitCell::getMillerIndex, UnitCell::getIntegerMillerIndex ! In that context, I introduced many new filters exploiting at best the possibility offered by this class. All of this triggered a huge refactoring of many parts of the code. Please review carefully and send me your feedback about this. That refactoring is probably an intermediate one as there is still some work to do with the duplicate locations for unit cells (i.e. Sample and Peak3D) and on a general point of view on the way to handle multiple cells for which we should perhaps rethink about at the view of the new workflow.