[la] VectorInterface::operator= broken

Using = on a vector object works, but not if accessed via the interface. I.e., the following code

using V = XT::LA::IstlDenseVector<double>;

V one(4, 1.);
std::cout << "one = " << print(one) << std::endl;
auto& one_as_if = static_cast<XT::LA::VectorInterface<typename V::Traits>&>(one);
std::cout << "one_as_if = " << print(one_as_if) << std::endl;

V two(4, 2.);
std::cout << "two = " << print(two) << std::endl;

one_as_if = two;
std::cout << "one_as_if = " << print(one_as_if) << std::endl;

one_as_if.as_imp() = two;
std::cout << "one_as_if = " << print(one_as_if) << std::endl;

gives the following output

one = [1, 1, 1, 1]
one_as_if = [1 1 1 1]
two = [2, 2, 2, 2]
one_as_if = [1 1 1 1] // <-
one_as_if = [2 2 2 2]

(except for the comment to mark the offending line). A one-shot debug attempt did not yield anything (the operator= was not even entered).

I do not really have time to pursue this atm, but I think this is a serious issue, possibly affecting the operator= of other CRTP interfaces. @l_tobi01 @r_milk01