Buggy behaviour of non-contiguous xadaptor and scalar assignment

[ewoudwempe]

Hi,
When assigning with a scalar on a xadaptor with non-contiguous strides, there is some problem where contiguous assignment is done instead, producing unexpected results.
For instance, the following does not work:

void test_strided_scalar_assign() {
  auto data = std::vector<double>(8);
  std::fill(data.begin(), data.end(), 0.);
  auto adapter_strided_noncont = xt::adapt(
      data.data(), 6, xt::no_ownership(), std::vector<size_t>{2, 3},
      std::vector<size_t>{4, 1});
  xt::noalias(adapter_strided_noncont) += 1;
  auto expected = std::vector<double>({1, 1, 1, 0, 1, 1, 1, 0});
  assert(expected == data);
}

When running this, the first 6 values of data are incremented, instead of taking into account the {4,1} strides.
What's do you think the best way is to fix this?