`ddc::print_dim_name` only handles `DiscreteDomain`

One cannot call `ddc::print_dim_name` on a set of elements different from `DiscreteDomain`. For example, the following code does not compile:

```cpp
struct X;
ddc::Chunk<int, ddc::StridedDiscreteDomain<X>> chk;
ddc::print_dim_name(std::cout, chk);
```