Test on static rank

I'm currently overloading based on rank. E.g.,
```cpp
void foo(xt::xtensor<double,2>& a);
void foo(xt::xtensor<double,4>& a);
```
I would like to make the API more flexible for example to use `xt::xtensor_fixed<double, xt::xshape<3, 3>>`. Is there a way to use SFINAE to overload based on rank, such that I can allow a very generic template?
