Move Ansatz type-hierarchy to a trait system

[mofeing]

⚠️ This is an idea I had that can be implemented in the future. We can experiment with it in other branch and decide on it in the future.

Currently, a Ansatz Tensor Network object cannot change its Ansatz type; you need to create a new object because the Ansatz is a part of its type (and the type of an object cannot be changed in Julia).

The motivation for being able to dynamically change the Ansatz of a Tensor Network is that during execution, operations performed into it can change its representation. Some examples are:

• (Mixed-)Canonization of a MPS/MPO
• MPS contraction of PEPS
• ...

In order to allow it, the way to go would be to stop the TensorNetwork type hierarchy in Ansatz, make Ansatz a concrete type and add a mutable field with the ansatz trait on it. The ansatz trait object type would be used for dynamic dispatch and its fields could be used for ansatz information (e.g. the orthogonality center in a mixed-canonized MPS).