Use function call operator for indexing instead of at

^{Originally posted by **bassoy** October 21, 2021}
Right now index accessing is performed using the `at` member function in tensor e.g. in [tensor_dynamic](https://github.com/boostorg/ublas/blob/develop/include/boost/numeric/ublas/tensor/tensor/tensor_dynamic.hpp), see also in [example](https://github.com/boostorg/ublas/blob/develop/examples/tensor/access_tensor.cpp).

```cpp
for(auto i = 0u; i < B.size(0); ++i)
for(auto j = 0u; j < B.size(1); ++j)
C.at(i,j) = std::conj(B.at(i,j));
```

Is it possible to overload `operator()` for this purpose? Note that `operator()` will also be used for creating subtensors.

```cpp
for(auto i = 0u; i < B.size(0); ++i)
for(auto j = 0u; j < B.size(1); ++j)
C(i,j) = std::conj(B(i,j));
```
We could also use `operator[]` for accessing single indices and multi-indices.
```cpp
for(auto i = 0u; i < B.size(0); ++i)
for(auto j = 0u; j < B.size(1); ++j)
C[i,j] = std::conj(B[i,j]);
```

This will be again closer to a Matlab or Octave or R notation.