This is an intermediate step towards removing HKL generation from CrystalStructure completely.

This will make some of the refactoring easier. Now CrystalStructure is doing manually what BraggScattererInCrystalStructure has been doing (creating equivalent positions), but this will be removed from CrystalStructure completely eventually.

Apparently VS does not like boost::make_shared as default argument in a constructor.

Putting those calculations in their own methods makes it easier to add other calculations later.

Things like unit cell, point group, lattice centering often occur together
and this class reflects their connection.