Tensor::slice(std::initializer_list<range1d<U>> ) is broken now

[shengg]

Tensor::slice() is broken, because it uses Tensorview constructor. However, explicit Tensorview(range_type&&, storage_type&) is private.

Since there is make_view(), maybe Tensor::slice is not useful now.

[shiozaki]

It's not clear to me if this function should return make_tensor or make_rwtensor. Maybe better to remove it.