Core: Peak::q returns invalid ReciprocalVector for a single image

    auto state = InterpolatedState::interpolate(_data->instrumentStates(), pixel_coords[2]);
    if (!state.isValid()) // this is the source of many interpolation problems
        return ReciprocalVector();

If state interpolation fails, Peak::q returns a default (invalid) ReciprocalVector. If there is only one detector image, then the interpolation will always fail.