Add free fermions support to lambeq circuits

[dimkart]

In a free-fermionic circuit, each box is replaced by a matchgate under the Jordan-Wigner transformation. This allows us to efficiently evaluate the diagram using covariance matrices and quadratic Hamiltonians.

See "Fermionic Gaussian states: an introduction to numerical approaches"

The natural place to add fermionic circuits support in lambeq is a new module in the backend package, e.g. fermionic.py. This module will be similar in form with backend.grammar, backend.tensor and backend.quantum, and will include:

• A new class backend.fermionic.Diagram which will inherit from backend.quantum.Diagram and override any parts that are specific to fermionic circuits. Parts that remain the same can come from the base class unaltered.
• New definitions for boxes (backend.fermionic.Box) and types (backend.fermionic.Ty), inheriting from the corresponding quantum classes, if required.
• New definitions for all quantum gates that change in the fermionic backend.
• New definitions for special boxes such as cups, caps, spiders, and swaps, when required.
• New eval() method, if required.
• New to_tn(), to_tk(), and to_pennylane() methods, to make training with all available training backends possible.