bugfix in Local_operator::apply
bugfix in Local_operator::apply

when applying the op to a single spin component, don't touch (zero) other
spin components in Wave_functions object.

electronic-structure/sirius,toxa81/sirius,electronic-structure/sirius,electronic-structure/sirius,toxa81/sirius,electronic-structure/sirius,toxa81/sirius,toxa81/sirius,electronic-structure/sirius,toxa81/sirius,toxa81/sirius,electronic-structure/sirius,electronic-structure/sirius