Organise converters and interfaces

[dimkart]

Currently lambeq interacts with and allows conversions to and from the packages tket, pennylane, and discopy. While the interface and the converters for tket and discopy are placed in the lambeq.backed.converters package, the interface+converters for pennylane is in lambeq.backend. All of these interfaces should be placed in the same position, preferably a dedicated package explicitly named interfaces inside backend.