Add Fermionic SWAP gate as a member of OpType enum.

[CalMacCQ]

The Fermionic SWAP gate has the unitary matrix

$$ \text{FSWAP} = \begin{pmatrix} 1 & 0 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & -1 \end{pmatrix} $$

You can implement this gate as Circuit(2).CZ(0, 1).SWAP(0, 1) or Circuit(2).SWAP(0, 1).CZ(0, 1).

Currently I'm handling the FSWAP gate as a CircBox containing a CZ and SWAP gate. Maybe we should add it as a pytket OpType for convienicence?

I thought previously that you could implement FSWAP as an OpType.FSim gate for some appropriate choice of angles. However it doesn't work out as nicely as I thought.

(Low priority)