Integration with Matrex

I can see two ways for integration:

1. "Tight" — when Tensorflex methods directly accept Matrex objects and return them.
2. "Loose" — when we use transfer methods, like `tensor_from_matrex/1` to derive tensors from Matrex objects.

My suggestion is to go with the loose method for now and to consider tight integration in some experimental branch.