investigate UnitCell::isEquivalent

UnitCell::isEquivalent checks for symmetry related hkl by applying element.getMatrix() to the hkl vector. Should this not be element.getRotationPart() instead? i.e., do we really want to apply the affine part of the transformation to the hkl vector when checking equivalence? @pellegrini @chapon could either of you give a justification why it should be so?