Use of Template Type in the Wrong Place

The template type, which is used for the type of extents harder coded to `std::size_t` instead of using the template type provided by the user. The user-provided type is misused to define the size of the static rank extents.

[Link](https://github.com/boostorg/ublas/blob/db29cdf76cdd4c3732164c99e9a22e9e02bf994f/include/boost/numeric/ublas/tensor/extents/extents_static_size.hpp#L41) to the bug.

Fix:

```cpp
template
class extents_core {
public:
using base_type = std::array;
....
}
```