Improving Subtensor Support for Tensor Library

The current subtensor does not support any tensor operations and some tensor functions due to time constraints. 

```cpp
auto t1 = tensor{dynamic_extents<>{1,2,3},1.f};'
auto t2 = tensor{dynamic_extents<>{1,2,3},1.f};
auto s1 = t1(slice<>{},slice<>{},slice<>{});
auto s2 = t2(slice<>{},slice<>{},slice<>{});
auto s3 = s1 + s2; // not valid
```